From 6182d4535435db9ff1de03e0adac457ca6f25edd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Bombeck?= <mbombeck@gmail.com>
Date: Tue, 20 Oct 2026 16:52:08 +0200
Subject: [PATCH] fix(i18n): print server-built sentences in the reader's
 notation

The decimal lint covered components and app only, and sentences for
people are built in src/lib too: the weekday weight correlation printed
"0.8 kg" into a German sentence, and the no-key fallback and the period
narrative's fallback spelled fractions with a full stop. Those three use
the new formatDecimal from format-locale with the reader's locale. The
rule now covers src/lib and src/hooks; text for the model, operator log
lines, SVG data, zone checks and wire values a client re-formats carry a
reasoned directive.
---
 .../__tests__/no-unlocalised-decimal.test.mjs | 20 ++++++++++++++++--
 .../healthlog/no-unlocalised-decimal.js       | 17 ++++++++++++---
 eslint.config.mjs                             |  3 ++-
 src/lib/ai/prompts/insight-system-prompt.ts   | 10 ++++++---
 src/lib/ai/prompts/shared-contracts.ts        |  2 ++
 src/lib/analytics/classifications.ts          |  2 ++
 src/lib/analytics/correlations-fast-path.ts   |  1 +
 src/lib/analytics/score/adiposity.ts          |  1 +
 src/lib/day/records.ts                        |  1 +
 src/lib/export/account-settings-backup.ts     |  1 +
 src/lib/export/backup-blob.ts                 |  1 +
 src/lib/format-locale.ts                      | 17 +++++++++++++++
 src/lib/import/unzip-export-xml.ts            |  2 ++
 .../insights/__tests__/correlations.test.ts   | 21 +++++++++++++++++++
 src/lib/insights/assessment-context.ts        |  1 +
 src/lib/insights/correlations.ts              |  7 +++++--
 src/lib/insights/mood-narratives.ts           |  7 +++++++
 .../period-narrative-deterministic.ts         | 12 ++++-------
 src/lib/insights/no-key-fallbacks.ts          | 21 +++++++++++--------
 src/lib/jobs/job-failures.ts                  |  3 ++-
 src/lib/jobs/offhost-backup.ts                |  1 +
 src/lib/labs/value-update.ts                  |  1 +
 src/lib/record-settings/configuration.ts      |  1 +
 src/lib/workouts/route-svg.ts                 |  2 ++
 24 files changed, 126 insertions(+), 29 deletions(-)

diff --git a/eslint-plugins/healthlog/__tests__/no-unlocalised-decimal.test.mjs b/eslint-plugins/healthlog/__tests__/no-unlocalised-decimal.test.mjs
index 1ef5dd6e4..89b164039 100644
--- a/eslint-plugins/healthlog/__tests__/no-unlocalised-decimal.test.mjs
+++ b/eslint-plugins/healthlog/__tests__/no-unlocalised-decimal.test.mjs
@@ -58,8 +58,12 @@ ruleTester.run("no-unlocalised-decimal", rule, {
       code: "const s = value.toFixed(2);",
       filename: "/repo/src/app/api/export/route.ts",
     },
-    // Outside the UI roots and in tests the rule is silent.
-    { code: "const s = value.toFixed(2);", filename: "/repo/src/lib/x.ts" },
+    // Outside the source roots and in tests the rule is silent.
+    { code: "const s = value.toFixed(2);", filename: "/repo/scripts/x.ts" },
+    {
+      code: "const s = value.toFixed(2);",
+      filename: "/repo/src/lib/__tests__/x.test.ts",
+    },
     {
       code: "const s = value.toFixed(2);",
       filename: "/repo/src/components/__tests__/x.test.tsx",
@@ -71,6 +75,18 @@ ruleTester.run("no-unlocalised-decimal", rule, {
       filename: APP_FILE,
       errors: [{ messageId: "fixed", data: { method: "toFixed" } }],
     },
+    // A sentence built on the server for a person: the weekday correlation
+    // printed "0.8 kg" in a German sentence from src/lib.
+    {
+      code: 'const delta = Math.abs(a - b).toFixed(1); t("x", { delta });',
+      filename: "/repo/src/lib/insights/correlations.ts",
+      errors: [{ messageId: "fixed" }],
+    },
+    {
+      code: "const s = value.toFixed(1);",
+      filename: "/repo/src/hooks/use-x.ts",
+      errors: [{ messageId: "fixed" }],
+    },
     {
       code: "const s = `${value.toFixed(1)} kg`;",
       filename: PAGE_FILE,
diff --git a/eslint-plugins/healthlog/no-unlocalised-decimal.js b/eslint-plugins/healthlog/no-unlocalised-decimal.js
index 51bb3f2cf..167a18009 100644
--- a/eslint-plugins/healthlog/no-unlocalised-decimal.js
+++ b/eslint-plugins/healthlog/no-unlocalised-decimal.js
@@ -18,8 +18,14 @@
  *   - rounding, not display: `Number(value.toFixed(2))` stays legal — the
  *     result is a number again and is formatted wherever it is shown
  *
- * FLAGGED, in `src/components/` and `src/app/` outside `src/app/api/` (wire
- * formats are not display) and outside test files:
+ * FLAGGED, in `src/components/`, `src/app/`, `src/hooks/` and `src/lib/`
+ * outside `src/app/api/` (wire formats are not display) and outside test
+ * files. `src/lib/` is in scope because sentences for people are built
+ * there too (a correlation hypothesis, a deterministic fallback, a narrative)
+ * and the weekday correlation printed "0.8 kg" into a German sentence; a
+ * machine-read string there (a prompt for the model, an operator log line,
+ * SVG path data, a wire value a client re-formats) carries a reasoned
+ * directive like any other.
  *
  *   - `<x>.toFixed(…)` and `<x>.toPrecision(…)`, unless the call is the
  *     argument of `Number(…)` / `parseFloat(…)` / `Number.parseFloat(…)` or
@@ -52,7 +58,12 @@
 "use strict";
 
 const RULE_NAME = "healthlog/no-unlocalised-decimal";
-const ENFORCED_ROOTS = ["src/components/", "src/app/"];
+const ENFORCED_ROOTS = [
+  "src/components/",
+  "src/app/",
+  "src/hooks/",
+  "src/lib/",
+];
 const EXEMPT_ROOTS = ["src/app/api/"];
 const MIN_REASON_CHARS = 8;
 
diff --git a/eslint.config.mjs b/eslint.config.mjs
index c6aabb847..a6bf8aac0 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -122,7 +122,8 @@ const eslintConfig = defineConfig([
       // planning-tags.js for the shape and its limits.
       "healthlog/no-planning-tags": "error",
       // A number shown to a person goes through useFormatters() (or
-      // makeFormatters outside React): toFixed() prints a full stop in every
+      // makeFormatters / formatDecimal outside React), in src/components,
+      // src/app, src/hooks and src/lib: toFixed() prints a full stop in every
       // language and a locale-less toLocaleString() follows the browser
       // instead of the language picked in the app. SVG path data and other
       // machine-read strings carry `eslint-disable-next-line ... -- <reason>`;
diff --git a/src/lib/ai/prompts/insight-system-prompt.ts b/src/lib/ai/prompts/insight-system-prompt.ts
index 75f8351d9..bd9c64675 100644
--- a/src/lib/ai/prompts/insight-system-prompt.ts
+++ b/src/lib/ai/prompts/insight-system-prompt.ts
@@ -106,18 +106,22 @@ export function buildComparisonBlock(
       return `- ${metric.type}: no prior-period data available`;
     }
     const sign = metric.delta > 0 ? "+" : metric.delta < 0 ? "-" : "±";
+    // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
     const formattedDelta = `${sign}${Math.abs(metric.delta).toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`;
     const pct =
       metric.deltaPercent != null
-        ? ` (${metric.deltaPercent > 0 ? "+" : metric.deltaPercent < 0 ? "-" : "±"}${Math.abs(metric.deltaPercent).toFixed(1)} %)`
+        ? // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
+          ` (${metric.deltaPercent > 0 ? "+" : metric.deltaPercent < 0 ? "-" : "±"}${Math.abs(metric.deltaPercent).toFixed(1)} %)`
         : "";
     const current =
       metric.currentAvg != null
-        ? `${metric.currentAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
+        ? // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
+          `${metric.currentAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
         : "n/a";
     const prior =
       metric.baselineAvg != null
-        ? `${metric.baselineAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
+        ? // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
+          `${metric.baselineAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
         : "n/a";
     return `- ${metric.type}: current ${current}, baseline ${prior}, delta ${formattedDelta}${pct}`;
   };
diff --git a/src/lib/ai/prompts/shared-contracts.ts b/src/lib/ai/prompts/shared-contracts.ts
index c80b08acb..ed957ea59 100644
--- a/src/lib/ai/prompts/shared-contracts.ts
+++ b/src/lib/ai/prompts/shared-contracts.ts
@@ -57,12 +57,14 @@ function deNum(value: number): string {
  * that only names 54 mg/dL cannot fire on it.
  */
 function glucoseFloor(mgdl: number, locale: ContractLocale): string {
+  // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
   const mmol = mgdlToMmol(mgdl).toFixed(1);
   return `${mgdl} mg/dL (${locale === "de" ? mmol.replace(".", ",") : mmol} mmol/L)`;
 }
 
 /** A temperature floor in both scales, °C first (the scale it is defined in). */
 function temperatureFloor(celsius: number, locale: ContractLocale): string {
+  // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
   const fahrenheit = (celsius * 1.8 + 32).toFixed(1);
   return locale === "de"
     ? `${deNum(celsius)} °C (${fahrenheit.replace(".", ",")} °F)`
diff --git a/src/lib/analytics/classifications.ts b/src/lib/analytics/classifications.ts
index 773d20872..7f9843d8e 100644
--- a/src/lib/analytics/classifications.ts
+++ b/src/lib/analytics/classifications.ts
@@ -486,12 +486,14 @@ export function generateAlerts(input: AlertInput): HealthAlert[] {
       alerts.push({
         level: "info",
         title: "Weight increasing",
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- English API alert text that no screen renders
         message: `Your weight is increasing by approx. ${weeklyChange.toFixed(1)} kg per week over the last 30 days.`,
       });
     } else if (weeklyChange < -0.5) {
       alerts.push({
         level: "info",
         title: "Weight decreasing",
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- English API alert text that no screen renders
         message: `Your weight is decreasing by approx. ${Math.abs(weeklyChange).toFixed(1)} kg per week over the last 30 days.`,
       });
     }
diff --git a/src/lib/analytics/correlations-fast-path.ts b/src/lib/analytics/correlations-fast-path.ts
index 80cee2633..f0c9d9e8d 100644
--- a/src/lib/analytics/correlations-fast-path.ts
+++ b/src/lib/analytics/correlations-fast-path.ts
@@ -359,6 +359,7 @@ export async function computeCorrelationHypothesesFastPath(
   const weightWeekday = correlateWeightWeekday(
     { daily: weightWeekdayPairs },
     t,
+    input.locale ?? defaultLocale,
   );
 
   annotate({
diff --git a/src/lib/analytics/score/adiposity.ts b/src/lib/analytics/score/adiposity.ts
index 6f0d370e3..53b73c071 100644
--- a/src/lib/analytics/score/adiposity.ts
+++ b/src/lib/analytics/score/adiposity.ts
@@ -81,6 +81,7 @@ export function computeAdiposityPillar(
       observed: {
         value: roundedRatio,
         unit: "ratio",
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire fallback label; the score detail prints the value through its own bundle key
         label: `waist-to-height ${roundedRatio.toFixed(3)}`,
         asOf: row.at.toISOString(),
         sources: [row.source],
diff --git a/src/lib/day/records.ts b/src/lib/day/records.ts
index 2d8d6cd11..d4794e2a8 100644
--- a/src/lib/day/records.ts
+++ b/src/lib/day/records.ts
@@ -713,6 +713,7 @@ async function labsPart(frame: DayFrame): Promise<SectionPart> {
           row.value !== null
             ? // Every recorded digit, without the float noise a converted
               // reading carries (the labs page prints the same figure).
+              // eslint-disable-next-line healthlog/no-unlocalised-decimal -- the day contract's meta is wire text; the lab figure keeps the notation it was stored in
               `${formatRecordedNumber(row.value, (v, digits) => v.toFixed(digits), row.sourceValue)} ${row.unit}`.trim()
             : (row.valueText ?? null),
         note: openText(row.noteEncrypted, "lab note"),
diff --git a/src/lib/export/account-settings-backup.ts b/src/lib/export/account-settings-backup.ts
index 2f7a426e5..2585a2c11 100644
--- a/src/lib/export/account-settings-backup.ts
+++ b/src/lib/export/account-settings-backup.ts
@@ -187,6 +187,7 @@ function isTimeZone(value: unknown): value is string {
     return false;
   }
   try {
+    // eslint-disable-next-line healthlog/no-unlocalised-decimal -- validates a zone name and formats nothing
     Intl.DateTimeFormat(undefined, { timeZone: value });
     return true;
   } catch {
diff --git a/src/lib/export/backup-blob.ts b/src/lib/export/backup-blob.ts
index 0a06a33a5..c7e2fbd7e 100644
--- a/src/lib/export/backup-blob.ts
+++ b/src/lib/export/backup-blob.ts
@@ -55,6 +55,7 @@ export function packBackupBlob(json: string): string {
 function size(bytes: number): string {
   const mb = bytes / 1024 / 1024;
   if (bytes < 1024 * 1024) return `${Math.round(bytes / 1024)} KB`;
+  // eslint-disable-next-line healthlog/no-unlocalised-decimal -- operator log text about a backup size
   return mb < 100 ? `${mb.toFixed(1)} MB` : `${Math.round(mb)} MB`;
 }
 
diff --git a/src/lib/format-locale.ts b/src/lib/format-locale.ts
index 76434a6fa..415f3e95c 100644
--- a/src/lib/format-locale.ts
+++ b/src/lib/format-locale.ts
@@ -179,6 +179,23 @@ export interface Formatters {
   monthShort: (value: DateInput) => string;
 }
 
+/**
+ * A decimal in the reader's notation for text built outside React (a server
+ * sentence, a deterministic fallback): at most `maxFractionDigits` digits,
+ * trailing zeros dropped, no grouping. `toFixed` prints a full stop in every
+ * language; this prints "0,8" in German and "0.8" in English.
+ */
+export function formatDecimal(
+  value: number,
+  locale: Locale,
+  maxFractionDigits: number,
+): string {
+  return getNumberFormat(resolveIntlLocale(locale), {
+    maximumFractionDigits: maxFractionDigits,
+    useGrouping: false,
+  }).format(value);
+}
+
 /**
  * Build the locale-aware formatter set.
  *
diff --git a/src/lib/import/unzip-export-xml.ts b/src/lib/import/unzip-export-xml.ts
index 3b28942a1..c90673a18 100644
--- a/src/lib/import/unzip-export-xml.ts
+++ b/src/lib/import/unzip-export-xml.ts
@@ -278,6 +278,7 @@ export async function extractExportXml(
         MAX_COMPRESSION_RATIO
     ) {
       throw new Error(
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- zero fraction digits: a whole ratio, no decimal mark to localise
         `export.xml advertises a ${(
           exportXmlEntry.uncompressedSize / exportXmlEntry.compressedSize
         ).toFixed(0)}× compression ratio (cap is ${MAX_COMPRESSION_RATIO}×)` +
@@ -399,6 +400,7 @@ export async function extractHealthConnectDb(
       entry.uncompressedSize / entry.compressedSize > MAX_COMPRESSION_RATIO
     ) {
       throw new Error(
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- zero fraction digits: a whole ratio, no decimal mark to localise
         `${HEALTH_CONNECT_DB_MEMBER} advertises a ${(
           entry.uncompressedSize / entry.compressedSize
         ).toFixed(0)}× compression ratio (cap is ${MAX_COMPRESSION_RATIO}×)` +
diff --git a/src/lib/insights/__tests__/correlations.test.ts b/src/lib/insights/__tests__/correlations.test.ts
index dee74ffc9..4caaab8fb 100644
--- a/src/lib/insights/__tests__/correlations.test.ts
+++ b/src/lib/insights/__tests__/correlations.test.ts
@@ -9,6 +9,7 @@ import {
   MAX_P_VALUE,
 } from "../correlations";
 import { seededRng } from "./helpers/seeded-series";
+import { getServerTranslator } from "@/lib/i18n/server-translator";
 
 /**
  * v1.4.20 — correlation discovery.
@@ -388,6 +389,26 @@ describe("correlateWeightWeekday", () => {
     }
   });
 
+  it("prints the difference in the reader's notation", () => {
+    const daily: Array<{ weekday: number; weight: number }> = [];
+    for (let week = 0; week < 4; week++) {
+      daily.push({ weekday: 0, weight: 84.0 + week * 0.1 });
+      for (let d = 1; d < 7; d++) daily.push({ weekday: d, weight: 80.0 });
+    }
+    const de = correlateWeightWeekday(
+      { daily },
+      getServerTranslator("de").t,
+      "de",
+    );
+    const en = correlateWeightWeekday({ daily });
+    if (de.status !== "ok" || en.status !== "ok") throw new Error("ok");
+    // 84.15 against a grand mean of 80.59: a 3.6 difference.
+    expect(de.interpretation).toMatch(/3,6/);
+    expect(de.interpretation).not.toMatch(/3\.6/);
+    // Counter-check: English keeps the full stop.
+    expect(en.interpretation).toMatch(/3\.6/);
+  });
+
   it("returns insufficient when no weekday meaningfully differs", () => {
     const daily = Array.from({ length: 28 }, (_, i) => ({
       weekday: i % 7,
diff --git a/src/lib/insights/assessment-context.ts b/src/lib/insights/assessment-context.ts
index 8cee9e9fb..cfc47795c 100644
--- a/src/lib/insights/assessment-context.ts
+++ b/src/lib/insights/assessment-context.ts
@@ -246,6 +246,7 @@ export function formatRelationsBlock(
   // Cap at two so the block never crowds out the metric's own grounding.
   const top = correlations.slice(0, 2);
   const lines = top.map(
+    // eslint-disable-next-line healthlog/no-unlocalised-decimal -- text for the model, never shown to a person
     (c) => `- ${c.interpretation} (n=${c.n}, r=${c.r.toFixed(2)})`,
   );
   if (locale === "en") {
diff --git a/src/lib/insights/correlations.ts b/src/lib/insights/correlations.ts
index 8a11eb58f..86ba3e7a9 100644
--- a/src/lib/insights/correlations.ts
+++ b/src/lib/insights/correlations.ts
@@ -39,7 +39,8 @@
  * the never-causal framing survives verbatim in all six locales rather than
  * leaking English into a non-English UI.
  */
-import { defaultLocale } from "@/lib/i18n/config";
+import { formatDecimal } from "@/lib/format-locale";
+import { defaultLocale, type Locale } from "@/lib/i18n/config";
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 import {
   dayKeyToIndex,
@@ -919,6 +920,7 @@ export interface WeightWeekdayInput {
 export function correlateWeightWeekday(
   input: WeightWeekdayInput,
   t: Translate = defaultTranslate(),
+  locale: Locale = defaultLocale,
 ): CorrelationResult {
   const points = input.daily.map((d) => ({ x: d.weekday, y: d.weight }));
   const anova = weekdayAnova(
@@ -951,7 +953,8 @@ export function correlateWeightWeekday(
   const direction = t(
     `insights.correlation.weekdayDirection.${outlierMean > grandMean ? "above" : "below"}`,
   );
-  const delta = Math.abs(outlierMean - grandMean).toFixed(1);
+  // In the reader's notation: the sentence is theirs ("0,8 kg" in German).
+  const delta = formatDecimal(Math.abs(outlierMean - grandMean), locale, 1);
   const interpretation = t("insights.correlation.hypothesis.weightWeekday", {
     weekday: dayLabel,
     delta,
diff --git a/src/lib/insights/mood-narratives.ts b/src/lib/insights/mood-narratives.ts
index a1fa24626..bf86593ae 100644
--- a/src/lib/insights/mood-narratives.ts
+++ b/src/lib/insights/mood-narratives.ts
@@ -158,6 +158,7 @@ function weekdayNarratives(input: MoodNarrativeInput): MoodNarrative[] {
       messageKey: "insights.mood.narrative.weekdayDip",
       vars: {
         weekdayKey: WEEKDAY_LABEL_KEYS[worst.weekday],
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
         delta: dipDelta.toFixed(1),
       },
       strength: dipDelta,
@@ -171,6 +172,7 @@ function weekdayNarratives(input: MoodNarrativeInput): MoodNarrative[] {
       messageKey: "insights.mood.narrative.weekdayPeak",
       vars: {
         weekdayKey: WEEKDAY_LABEL_KEYS[best.weekday],
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
         value: best.avgScore.toFixed(1),
       },
       strength: peakDelta,
@@ -208,6 +210,7 @@ function trendNarrative(input: MoodNarrativeInput): MoodNarrative | null {
         : "insights.mood.narrative.trendDown",
     vars: {
       direction: delta > 0 ? "up" : "down",
+      // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
       delta: Math.abs(delta).toFixed(1),
       days: String(w),
     },
@@ -248,6 +251,7 @@ function weekendNarrative(input: MoodNarrativeInput): MoodNarrative | null {
         : "insights.mood.narrative.weekendDown",
     vars: {
       direction: delta > 0 ? "up" : "down",
+      // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
       delta: Math.abs(delta).toFixed(1),
     },
     strength: Math.abs(delta),
@@ -321,6 +325,7 @@ function tagNarratives(input: MoodNarrativeInput): MoodNarrative[] {
     out.push({
       kind: "tag-lift",
       messageKey: "insights.mood.narrative.tagLift",
+      // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
       vars: { ...tagVars(topLift), delta: topLift.delta.toFixed(1) },
       strength: topLift.delta,
     });
@@ -332,6 +337,7 @@ function tagNarratives(input: MoodNarrativeInput): MoodNarrative[] {
       messageKey: "insights.mood.narrative.tagDrop",
       vars: {
         ...tagVars(topDrop),
+        // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
         delta: Math.abs(topDrop.delta).toFixed(1),
       },
       strength: Math.abs(topDrop.delta),
@@ -427,6 +433,7 @@ function timeOfDayNarrative(input: MoodNarrativeInput): MoodNarrative | null {
     messageKey: "insights.mood.narrative.timeOfDay",
     vars: {
       bucketKey: TIME_OF_DAY_LABEL_KEYS[best],
+      // eslint-disable-next-line healthlog/no-unlocalised-decimal -- wire value; the narrative feed parses it and prints it through useFormatters
       value: bestRow.avgScore.toFixed(1),
     },
     strength: delta,
diff --git a/src/lib/insights/narrative/period-narrative-deterministic.ts b/src/lib/insights/narrative/period-narrative-deterministic.ts
index 9f85bb259..92d0c613f 100644
--- a/src/lib/insights/narrative/period-narrative-deterministic.ts
+++ b/src/lib/insights/narrative/period-narrative-deterministic.ts
@@ -17,6 +17,7 @@
  * — with a prettified-enum fallback for any unmapped type, so a newly added
  * measurement type degrades gracefully instead of leaking an enum constant.
  */
+import { formatDecimal } from "@/lib/format-locale";
 import type { PeriodNarrativeContext, MetricDelta } from "./period-narrative";
 
 /** Marker stored in `InsightNarrative.providerType` for a fallback row. */
@@ -79,14 +80,9 @@ function labelFor(type: string, locale: Locale): string {
 /** Format a signed number with the locale's decimal mark (no thousands sep). */
 function fmtSigned(n: number, locale: Locale): string {
   const sign = n > 0 ? "+" : n < 0 ? "−" : "±"; // U+2212 minus for typography
-  const abs = Math.abs(n);
-  // Integers print verbatim; fractions print at 2dp with trailing zeros
-  // trimmed (so 1.20 → 1.2, 1.50 → 1.5) WITHOUT touching integer zeros.
-  const s = Number.isInteger(abs)
-    ? String(abs)
-    : abs.toFixed(2).replace(/0+$/, "").replace(/\.$/, "");
-  const localised = locale === "de" ? s.replace(".", ",") : s;
-  return `${sign}${localised}`;
+  // At most two decimals, trailing zeros trimmed (1.20 → 1.2), in the
+  // reader's notation in every language, not only German.
+  return `${sign}${formatDecimal(Math.abs(n), locale, 2)}`;
 }
 
 /** A delta is "notable" when it has a computable, non-zero change. */
diff --git a/src/lib/insights/no-key-fallbacks.ts b/src/lib/insights/no-key-fallbacks.ts
index fc3fc42a1..259d71e25 100644
--- a/src/lib/insights/no-key-fallbacks.ts
+++ b/src/lib/insights/no-key-fallbacks.ts
@@ -1,4 +1,5 @@
 import type { Locale } from "@/lib/i18n/config";
+import { formatDecimal } from "@/lib/format-locale";
 import type { MetricSignal } from "@/lib/insights/metric-signal";
 
 /**
@@ -66,16 +67,18 @@ interface FallbackCopy {
   pointer: Localized<string>;
 }
 
-function fmtValue(value: number, copy: FallbackCopy): string {
+function fmtValue(
+  value: number,
+  copy: FallbackCopy,
+  locale: InsightLocale,
+): string {
   const digits = copy.digits ?? 0;
-  // Round to the metric's precision, but never render a trailing ".0" — an
-  // integer reading (steps, a whole-number BMI day) should read as "4200",
-  // not "4200.0", while a genuine fractional value keeps its decimal.
+  // Round to the metric's precision, but never render a trailing zero — an
+  // integer reading (steps, a whole-number BMI day) reads as "4200", not
+  // "4200.0" — and print the decimal in the reader's notation ("72,5").
   const factor = 10 ** digits;
   const rounded = Math.round(value * factor) / factor;
-  const text = Number.isInteger(rounded)
-    ? String(rounded)
-    : rounded.toFixed(digits);
+  const text = formatDecimal(rounded, locale, digits);
   return copy.unit ? `${text}${copy.unit}` : text;
 }
 
@@ -255,7 +258,7 @@ function composeGroundedFallback(
   if (!signal || !Number.isFinite(signal.current)) return null;
 
   const label = capitalise(pick(locale, copy.label));
-  const current = fmtValue(signal.current, copy);
+  const current = fmtValue(signal.current, copy, locale);
 
   const sentences: string[] = [];
 
@@ -271,7 +274,7 @@ function composeGroundedFallback(
     signal.delta !== 0 &&
     Number.isFinite(signal.baseline)
   ) {
-    const baseline = fmtValue(signal.baseline, copy);
+    const baseline = fmtValue(signal.baseline, copy, locale);
     sentences.push(
       pick(locale, VALUE_VS_BASELINE)(
         label,
diff --git a/src/lib/jobs/job-failures.ts b/src/lib/jobs/job-failures.ts
index db7604b6d..3b4b8f004 100644
--- a/src/lib/jobs/job-failures.ts
+++ b/src/lib/jobs/job-failures.ts
@@ -129,7 +129,8 @@ function explainTimeout(message: string, expireSeconds: number | null): string {
   const limit =
     expireSeconds !== null && expireSeconds > 0
       ? expireSeconds >= 3600 && expireSeconds % 3600 === 0
-        ? `${expireSeconds / 3600} h`
+        ? // eslint-disable-next-line healthlog/no-unlocalised-decimal -- operator log text, a whole number of hours (checked above)
+          `${expireSeconds / 3600} h`
         : `${Math.round(expireSeconds / 60)} min`
       : null;
   return (
diff --git a/src/lib/jobs/offhost-backup.ts b/src/lib/jobs/offhost-backup.ts
index 05a9f23b5..1016ce032 100644
--- a/src/lib/jobs/offhost-backup.ts
+++ b/src/lib/jobs/offhost-backup.ts
@@ -425,6 +425,7 @@ export function defaultOffhostObjectLimit(): number {
 function size(bytes: number): string {
   const mb = bytes / 1024 / 1024;
   if (bytes < 1024 * 1024) return `${Math.round(bytes / 1024)} KB`;
+  // eslint-disable-next-line healthlog/no-unlocalised-decimal -- operator log text about a backup size
   return mb < 100 ? `${mb.toFixed(1)} MB` : `${Math.round(mb)} MB`;
 }
 
diff --git a/src/lib/labs/value-update.ts b/src/lib/labs/value-update.ts
index 7dd945dc5..d0bdd4b81 100644
--- a/src/lib/labs/value-update.ts
+++ b/src/lib/labs/value-update.ts
@@ -17,6 +17,7 @@ export function valueEditSeed(value: number): string {
     MAX_SEED_DECIMALS,
     Math.max(DISPLAY_DECIMALS, SEED_SIGNIFICANT_FIGURES - 1 - magnitude),
   );
+  // eslint-disable-next-line healthlog/no-unlocalised-decimal -- seeds an input whose parser reads the full stop back
   return value
     .toFixed(decimals)
     .replace(/(\.\d*?)0+$/, "$1")
diff --git a/src/lib/record-settings/configuration.ts b/src/lib/record-settings/configuration.ts
index 632460ef9..0ebc84a24 100644
--- a/src/lib/record-settings/configuration.ts
+++ b/src/lib/record-settings/configuration.ts
@@ -46,6 +46,7 @@ const timezoneSchema = z
   .max(64)
   .refine((timezone) => {
     try {
+      // eslint-disable-next-line healthlog/no-unlocalised-decimal -- validates a zone name and formats nothing
       Intl.DateTimeFormat(undefined, { timeZone: timezone });
       return true;
     } catch {
diff --git a/src/lib/workouts/route-svg.ts b/src/lib/workouts/route-svg.ts
index 23d6d831c..2c4d29952 100644
--- a/src/lib/workouts/route-svg.ts
+++ b/src/lib/workouts/route-svg.ts
@@ -250,11 +250,13 @@ export function projectRoute(
 
   const vbPoints = simplified.map(toVB);
   const path = vbPoints
+    // eslint-disable-next-line healthlog/no-unlocalised-decimal -- SVG path data and viewBox, read by the renderer
     .map((p, i) => `${i === 0 ? "M" : "L"}${p.x.toFixed(2)} ${p.y.toFixed(2)}`)
     .join(" ");
 
   return {
     path,
+    // eslint-disable-next-line healthlog/no-unlocalised-decimal -- SVG path data and viewBox, read by the renderer
     viewBox: `0 0 ${VIEW_W} ${height.toFixed(2)}`,
     width: VIEW_W,
     height,