diff --git a/Dockerfile b/Dockerfile
index ecc726d6a..db86834e2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -60,6 +60,11 @@ RUN node -p "require('/app/node_modules/pdfjs-dist/package.json').version" \
 # The same for @napi-rs/canvas, whose native binary must match its JS loader.
 RUN node -p "require('/app/node_modules/@napi-rs/canvas/package.json').version" \
       > /app/.canvas-version
+# Every shipped locale's message catalog is present and parses, checked
+# against the locale list in the source; the list is written out so the runner
+# stage can check the catalogs the image actually carries.
+RUN node scripts/verify-message-catalogs.mjs --dir messages \
+      --config src/lib/i18n/config.ts --write /app/.message-locales
 
 # ── Stage 3: Production runner ─────────────────────────────
 FROM node:22-alpine@sha256:16e22a550f3863206a3f701448c45f7912c6896a62de43add43bb9c86130c3e2 AS runner
@@ -111,6 +116,12 @@ COPY --from=builder /app/public ./public
 # The server reads its message catalogs from here on first use, one locale at
 # a time (src/lib/i18n/server-catalogs.ts); they are not bundled.
 COPY --from=builder /app/messages ./messages
+# A missing or broken catalog is only noticed when a page asks for that
+# locale, so check the copies in this stage against the builder's list.
+COPY --from=builder /app/.message-locales /app/scripts/verify-message-catalogs.mjs /tmp/catalog-check/
+RUN node /tmp/catalog-check/verify-message-catalogs.mjs --dir /app/messages \
+      --locales /tmp/catalog-check/.message-locales && \
+    rm -rf /tmp/catalog-check
 COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
 COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
 # Next's standalone output must carry the externalized worker dependencies.
diff --git a/scripts/verify-message-catalogs.mjs b/scripts/verify-message-catalogs.mjs
new file mode 100644
index 000000000..0e8dd79b5
--- /dev/null
+++ b/scripts/verify-message-catalogs.mjs
@@ -0,0 +1,98 @@
+#!/usr/bin/env node
+/**
+ * Checks that every shipped locale has a message catalog that parses.
+ *
+ * The server reads `messages/<locale>.json` from disk at run time
+ * (`src/lib/i18n/server-catalogs.ts`); nothing is bundled, so an image that
+ * lost the directory, or one file of it, builds and starts clean and only
+ * shows the gap when a page asks for that locale. The Dockerfile runs this
+ * twice: in the builder against the locale list in `src/lib/i18n/config.ts`,
+ * writing that list out, and in the runner stage against the catalogs the
+ * image actually carries.
+ *
+ *   node scripts/verify-message-catalogs.mjs --dir messages \
+ *     --config src/lib/i18n/config.ts [--write .message-locales]
+ *   node verify-message-catalogs.mjs --dir /app/messages \
+ *     --locales .message-locales
+ *
+ * Exits 1 with every problem listed when a catalog is missing, does not parse
+ * or is not a JSON object, or when the locale list cannot be read.
+ */
+import { readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+
+function arg(name) {
+  const index = process.argv.indexOf(`--${name}`);
+  return index === -1 ? null : (process.argv[index + 1] ?? null);
+}
+
+function localesFromConfig(path) {
+  const source = readFileSync(path, "utf8");
+  const block = /export const locales = \[([^\]]*)\]/.exec(source);
+  if (!block) throw new Error(`no \`export const locales = [...]\` in ${path}`);
+  const list = [...block[1].matchAll(/"([^"]+)"/g)].map((m) => m[1]);
+  if (list.length === 0) throw new Error(`the locale list in ${path} is empty`);
+  return list;
+}
+
+function localesFromFile(path) {
+  const list = readFileSync(path, "utf8").split("\n").filter(Boolean);
+  if (list.length === 0) throw new Error(`the locale list in ${path} is empty`);
+  return list;
+}
+
+const dir = arg("dir");
+const config = arg("config");
+const listFile = arg("locales");
+if (!dir || (!config && !listFile)) {
+  console.error(
+    "usage: verify-message-catalogs.mjs --dir <messages> (--config <config.ts> | --locales <file>) [--write <file>]",
+  );
+  process.exit(2);
+}
+
+let locales;
+try {
+  locales = config ? localesFromConfig(config) : localesFromFile(listFile);
+} catch (error) {
+  console.error(`ERROR: ${error.message}`);
+  process.exit(1);
+}
+
+const problems = [];
+for (const locale of locales) {
+  const path = join(dir, `${locale}.json`);
+  let text;
+  try {
+    text = readFileSync(path, "utf8");
+  } catch (error) {
+    problems.push(
+      `${path}: ${error.code === "ENOENT" ? "missing" : error.message}`,
+    );
+    continue;
+  }
+  try {
+    const parsed = JSON.parse(text);
+    if (
+      parsed === null ||
+      typeof parsed !== "object" ||
+      Array.isArray(parsed)
+    ) {
+      problems.push(`${path}: not a JSON object`);
+    }
+  } catch (error) {
+    problems.push(`${path}: does not parse (${error.message})`);
+  }
+}
+
+if (problems.length > 0) {
+  console.error(
+    "ERROR: message catalogs the server reads at run time are broken:",
+  );
+  for (const problem of problems) console.error(`  ${problem}`);
+  process.exit(1);
+}
+
+const write = arg("write");
+if (write) writeFileSync(write, `${locales.join("\n")}\n`);
+console.log(`message catalogs ok: ${locales.join(", ")} in ${dir}`);
diff --git a/src/__tests__/verify-message-catalogs-script.test.ts b/src/__tests__/verify-message-catalogs-script.test.ts
new file mode 100644
index 000000000..06628fb27
--- /dev/null
+++ b/src/__tests__/verify-message-catalogs-script.test.ts
@@ -0,0 +1,63 @@
+/**
+ * `scripts/verify-message-catalogs.mjs`, which the image build runs in the
+ * builder and again in the runner stage, passes the shipped catalogs and fails
+ * on a missing or corrupt one. A check that cannot fail would let an image
+ * without its catalogs through, so both directions are pinned.
+ */
+import { spawnSync } from "node:child_process";
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { afterEach, describe, expect, it } from "vitest";
+
+const SCRIPT = join(process.cwd(), "scripts", "verify-message-catalogs.mjs");
+const CONFIG = join(process.cwd(), "src", "lib", "i18n", "config.ts");
+
+function run(args: string[]) {
+  return spawnSync(process.execPath, [SCRIPT, ...args], { encoding: "utf8" });
+}
+
+let scratch: string | null = null;
+afterEach(() => {
+  if (scratch) rmSync(scratch, { recursive: true, force: true });
+  scratch = null;
+});
+
+describe("verify-message-catalogs", () => {
+  it("passes the catalogs in the tree and writes the locale list", () => {
+    scratch = mkdtempSync(join(tmpdir(), "hl-catalog-check-"));
+    const list = join(scratch, "locales");
+    const res = run([
+      "--dir",
+      join(process.cwd(), "messages"),
+      "--config",
+      CONFIG,
+      "--write",
+      list,
+    ]);
+    expect(res.status).toBe(0);
+    expect(res.stdout).toMatch(/message catalogs ok: .*\ben\b/);
+
+    // The runner-stage form, against the list the builder wrote.
+    expect(
+      run(["--dir", join(process.cwd(), "messages"), "--locales", list]).status,
+    ).toBe(0);
+  });
+
+  it("fails on a missing catalog and on one that does not parse", () => {
+    scratch = mkdtempSync(join(tmpdir(), "hl-catalog-check-"));
+    const dir = join(scratch, "messages");
+    mkdirSync(dir);
+    writeFileSync(join(dir, "en.json"), "{}");
+    writeFileSync(join(dir, "de.json"), '{"common": ');
+    const list = join(scratch, "locales");
+    writeFileSync(list, "en\nde\nfr\n");
+
+    const res = run(["--dir", dir, "--locales", list]);
+    expect(res.status).toBe(1);
+    expect(res.stderr).toMatch(/de\.json: does not parse/);
+    expect(res.stderr).toMatch(/fr\.json: missing/);
+    expect(res.stderr).not.toMatch(/en\.json/);
+  });
+});
diff --git a/src/instrumentation.ts b/src/instrumentation.ts
index 35eddfef6..d75f9f8e5 100644
--- a/src/instrumentation.ts
+++ b/src/instrumentation.ts
@@ -13,6 +13,14 @@ export async function register() {
       ],
     );
     startEventLoopLagMonitor();
+
+    // Every locale falls back to the English message catalog, so a process
+    // without a readable `messages/en.json` cannot serve a page or a job.
+    // Refuse to start with the file named, rather than fail every request.
+    const { assertDefaultCatalogReadable } =
+      await import("@/lib/i18n/server-catalogs");
+    assertDefaultCatalogReadable();
+
     beginBootStep("readiness_summary");
 
     // First-boot readiness summary: a human-readable block on the core secrets
diff --git a/src/lib/i18n/__tests__/server-catalogs-fallback.test.ts b/src/lib/i18n/__tests__/server-catalogs-fallback.test.ts
new file mode 100644
index 000000000..cd709fccf
--- /dev/null
+++ b/src/lib/i18n/__tests__/server-catalogs-fallback.test.ts
@@ -0,0 +1,91 @@
+/**
+ * A message catalog that cannot be read costs its own locale, not every
+ * request.
+ *
+ * The catalogs are read from `messages/` at run time. A missing or corrupt
+ * file used to throw from `serverCatalog`, and because the matchers iterate
+ * every locale (`localisedValues`), one bad non-English file made every
+ * request that touched them a 500. Now that locale is served the English
+ * catalog and the failure is logged once; the English catalog itself, which
+ * everything falls back to, stops the process at boot with the file named.
+ */
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/logging/signal", () => ({ emitSignal: vi.fn() }));
+
+import { emitSignal } from "@/lib/logging/signal";
+
+import { localisedValues } from "../shared-resolve";
+import {
+  assertDefaultCatalogReadable,
+  serverCatalog,
+} from "../server-catalogs";
+
+const CACHE = Symbol.for("healthlog.i18n.serverCatalogs");
+const EN = { common: { save: "Save" } };
+
+function clearCatalogCache() {
+  (globalThis as { [CACHE]?: Map<string, unknown> })[CACHE]?.clear();
+}
+
+let root: string;
+
+beforeEach(() => {
+  root = mkdtempSync(join(tmpdir(), "hl-catalogs-"));
+  mkdirSync(join(root, "messages"));
+  writeFileSync(join(root, "messages", "en.json"), JSON.stringify(EN));
+  writeFileSync(join(root, "messages", "de.json"), '{"common": {"save": ');
+  vi.spyOn(process, "cwd").mockReturnValue(root);
+  vi.mocked(emitSignal).mockClear();
+  clearCatalogCache();
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  clearCatalogCache();
+  rmSync(root, { recursive: true, force: true });
+});
+
+describe("a catalog that cannot be read", () => {
+  it("serves a corrupt locale the English catalog and logs it once", () => {
+    expect(serverCatalog("de")).toEqual(EN);
+    expect(serverCatalog("de")).toEqual(EN);
+    expect(emitSignal).toHaveBeenCalledTimes(1);
+    expect(emitSignal).toHaveBeenCalledWith(
+      expect.objectContaining({
+        action: "i18n.catalog.fallback",
+        level: "warn",
+        meta: { locale: "de", reason: "corrupt", fallback: "en" },
+      }),
+    );
+  });
+
+  it("serves a missing locale the English catalog", () => {
+    expect(serverCatalog("fr")).toEqual(EN);
+    expect(emitSignal).toHaveBeenCalledWith(
+      expect.objectContaining({
+        meta: { locale: "fr", reason: "missing", fallback: "en" },
+      }),
+    );
+  });
+
+  it("keeps a matcher that reads every locale answering", () => {
+    expect(localisedValues("common.save")).toEqual(["Save"]);
+  });
+
+  it("refuses to boot without the English catalog, naming the file", () => {
+    rmSync(join(root, "messages", "en.json"));
+    expect(() => assertDefaultCatalogReadable()).toThrow(
+      /en message catalog at .*messages[/\\]en\.json is missing/,
+    );
+  });
+
+  it("refuses to boot on an English catalog that does not parse", () => {
+    writeFileSync(join(root, "messages", "en.json"), "[1, 2");
+    expect(() => assertDefaultCatalogReadable()).toThrow(/is corrupt/);
+  });
+});
diff --git a/src/lib/i18n/server-catalogs.ts b/src/lib/i18n/server-catalogs.ts
index cd9f1b555..138cd9f29 100644
--- a/src/lib/i18n/server-catalogs.ts
+++ b/src/lib/i18n/server-catalogs.ts
@@ -18,12 +18,23 @@
  * milliseconds.
  *
  * The directory is `messages/` under the working directory: the checkout in
- * development and tests, `/app` in the image, which copies it there.
+ * development and tests, `/app` in the image, which copies it there (and the
+ * image build checks every locale's file is there and parses).
+ *
+ * A catalog that cannot be read is not every request's problem. A missing or
+ * corrupt non-English catalog is logged once per process and that locale is
+ * served the English one; before, the read threw, and since the matchers
+ * iterate every locale, one bad file turned every request into a 500. The
+ * English catalog is the fallback itself, so a process without it cannot serve
+ * at all: the boot check (`assertDefaultCatalogReadable`) refuses to start one
+ * with a message that names the file.
  */
 import { readFileSync } from "node:fs";
 import { join } from "node:path";
 
-import { locales, type Locale } from "./config";
+import { emitSignal } from "@/lib/logging/signal";
+
+import { defaultLocale, locales, type Locale } from "./config";
 
 export type ServerCatalog = Record<string, unknown>;
 
@@ -51,11 +62,87 @@ export function serverCatalog(locale: Locale): ServerCatalog {
   if (!(locales as readonly string[]).includes(locale)) {
     throw new Error(`unknown locale: ${String(locale)}`);
   }
-  const parsed = JSON.parse(
-    readFileSync(join(catalogDirectory(), `${locale}.json`), "utf8"),
-  ) as ServerCatalog;
-  loaded.set(locale, parsed);
-  return parsed;
+  const read = readCatalog(locale);
+  if (read.ok) {
+    loaded.set(locale, read.catalog);
+    return read.catalog;
+  }
+  if (locale === defaultLocale) {
+    throw new Error(defaultCatalogMessage(read));
+  }
+  // Logged once: the fallback is cached under the locale, so the next read
+  // of it is a cache hit.
+  emitSignal({
+    action: "i18n.catalog.fallback",
+    level: "warn",
+    meta: { locale, reason: read.reason, fallback: defaultLocale },
+    error: read.error,
+  });
+  const fallback = serverCatalog(defaultLocale);
+  loaded.set(locale, fallback);
+  return fallback;
+}
+
+type CatalogRead =
+  | { ok: true; catalog: ServerCatalog }
+  | {
+      ok: false;
+      path: string;
+      reason: "missing" | "unreadable" | "corrupt";
+      error: unknown;
+    };
+
+function readCatalog(locale: Locale): CatalogRead {
+  const path = join(catalogDirectory(), `${locale}.json`);
+  let text: string;
+  try {
+    text = readFileSync(path, "utf8");
+  } catch (error) {
+    const missing = (error as NodeJS.ErrnoException)?.code === "ENOENT";
+    return {
+      ok: false,
+      path,
+      reason: missing ? "missing" : "unreadable",
+      error,
+    };
+  }
+  try {
+    const parsed: unknown = JSON.parse(text);
+    if (
+      parsed === null ||
+      typeof parsed !== "object" ||
+      Array.isArray(parsed)
+    ) {
+      return {
+        ok: false,
+        path,
+        reason: "corrupt",
+        error: new Error("the catalog is not a JSON object"),
+      };
+    }
+    return { ok: true, catalog: parsed as ServerCatalog };
+  } catch (error) {
+    return { ok: false, path, reason: "corrupt", error };
+  }
+}
+
+function defaultCatalogMessage(read: Extract<CatalogRead, { ok: false }>) {
+  const detail =
+    read.error instanceof Error ? read.error.message : String(read.error);
+  return (
+    `The ${defaultLocale} message catalog at ${read.path} is ${read.reason} ` +
+    `(${detail}). Every other locale falls back to it, so the server cannot ` +
+    `serve without it; the image must ship messages/*.json next to server.js.`
+  );
+}
+
+/**
+ * Boot check: read the English catalog now and throw a message naming the
+ * file when it is missing or does not parse, so a broken image stops at boot
+ * instead of failing every request.
+ */
+export function assertDefaultCatalogReadable(): void {
+  serverCatalog(defaultLocale);
 }
 
 /** Locales whose catalog this process has read so far (for tests and logs). */