diff --git a/public/sw.js b/public/sw.js
index 8c8141415..c1de63368 100644
--- a/public/sw.js
+++ b/public/sw.js
@@ -168,6 +168,17 @@ self.addEventListener("fetch", (event) => {
     return;
   }
 
+  // Locale-catalog boot script — cache-first. The URL is versioned
+  // (`/i18n/<locale>?v=<build>`) and served immutable, so cache-first is
+  // correct and keeps the catalog available on an offline relaunch (the
+  // cached shell HTML references the same versioned URL it was rendered
+  // with). Carries no user data — same public strings as the repository's
+  // messages/*.json.
+  if (url.pathname.startsWith("/i18n/")) {
+    event.respondWith(cacheFirst(request, STATIC_CACHE));
+    return;
+  }
+
   // Static files (fonts, icons, images) — cache-first with long TTL
   if (/\.(png|svg|jpg|jpeg|gif|webp|ico|woff2?|ttf|eot)$/i.test(url.pathname)) {
     event.respondWith(cacheFirst(request, STATIC_CACHE));
diff --git a/src/app/i18n/[locale]/route.ts b/src/app/i18n/[locale]/route.ts
new file mode 100644
index 000000000..244829fcb
--- /dev/null
+++ b/src/app/i18n/[locale]/route.ts
@@ -0,0 +1,51 @@
+import { NextResponse } from "next/server";
+
+import { locales, type Locale } from "@/lib/i18n/config";
+import { allMessages } from "@/lib/i18n/shared-resolve";
+
+/**
+ * Locale-catalog boot script.
+ *
+ * The root layout emits `<script src="/i18n/<locale>?v=<version>" defer>`
+ * ahead of Next's hydration scripts. The body assigns the active locale's
+ * full message bundle to `self.__HL_I18N`, which `load-locale.ts` reads in
+ * its browser cache initializer — so the first client render holds the same
+ * strings the server rendered with.
+ *
+ * This replaces the former RSC-prop handoff that serialized the ENTIRE
+ * active catalog into every document's flight payload (392 KB of a 505 KB
+ * dashboard HTML, re-downloaded on every hard load). As a versioned URL the
+ * catalog is immutable-cacheable: one download per deploy per locale, then
+ * HTTP/SW cache. The `?v=` query is only a cache key; the handler always
+ * serves the catalog baked into the running build.
+ *
+ * Public by design (allowlisted in `src/proxy.ts`): the login page needs it
+ * pre-auth, and the catalogs are the same strings that ship in the public
+ * repository — no tenant data, no secrets.
+ */
+
+const IMMUTABLE_CACHE = "public, max-age=31536000, immutable";
+
+export async function GET(
+  _request: Request,
+  { params }: { params: Promise<{ locale: string }> },
+) {
+  const { locale } = await params;
+  if (!(locales as readonly string[]).includes(locale)) {
+    return new NextResponse(null, { status: 404 });
+  }
+
+  const payload = JSON.stringify({
+    locale,
+    messages: allMessages[locale as Locale],
+  });
+  return new NextResponse(`self.__HL_I18N = ${payload};`, {
+    status: 200,
+    headers: {
+      "Content-Type": "text/javascript; charset=utf-8",
+      "Cache-Control": IMMUTABLE_CACHE,
+      // Belt and suspenders for a script-typed response.
+      "X-Content-Type-Options": "nosniff",
+    },
+  });
+}
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 0643c3cba..c7d841720 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -7,7 +7,6 @@ import { AuthShell } from "@/components/layout/auth-shell";
 import { MonitoringBootstrap } from "@/components/monitoring/bootstrap";
 import { WebVitalsReporter } from "@/components/monitoring/web-vitals-reporter";
 import { resolveInitialLocale } from "@/lib/i18n/resolve-initial-locale";
-import { allMessages } from "@/lib/i18n/shared-resolve";
 
 const inter = Inter({
   variable: "--font-sans",
@@ -89,13 +88,6 @@ export default async function RootLayout({
       : undefined;
 
   const initialLocale = await resolveInitialLocale();
-  // RSC handoff for the i18n bundle split: only EN ships statically in
-  // the client chunk, so a non-EN first paint needs its bundle inlined
-  // into the payload here — that is what keeps the split free of the
-  // EN→DE hydration flash. EN passes nothing (the client already holds
-  // the static fallback floor).
-  const initialMessages =
-    initialLocale === "en" ? undefined : allMessages[initialLocale];
 
   return (
     <html lang={initialLocale} suppressHydrationWarning>
@@ -105,12 +97,26 @@ export default async function RootLayout({
           nonce={nonce}
           dangerouslySetInnerHTML={{ __html: themeScript }}
         />
+        {/*
+          Locale-catalog boot script (see src/app/i18n/[locale]/route.ts).
+          Replaces the former RSC-prop handoff that inlined the whole
+          active catalog into every document's flight payload (392 KB of a
+          505 KB dashboard HTML). `defer` keeps it off the parse/paint
+          critical path while still executing before Next's hydration
+          module scripts (both ride the after-parse in-order queue, and
+          this tag comes first), so `load-locale.ts` finds `self.__HL_I18N`
+          set when the client bundle initializes — SSR text and first
+          client render stay identical. The `?v=` build-version key makes
+          the response immutable-cacheable: one catalog download per
+          deploy, then HTTP/SW cache instead of per-document payload.
+        */}
+        <script
+          src={`/i18n/${initialLocale}?v=${process.env.NEXT_PUBLIC_APP_VERSION ?? "dev"}`}
+          defer
+        />
       </head>
       <body className={`${inter.variable} font-sans antialiased`}>
-        <Providers
-          initialLocale={initialLocale}
-          initialMessages={initialMessages}
-        >
+        <Providers initialLocale={initialLocale}>
           <MonitoringBootstrap />
           <WebVitalsReporter />
           {/* `DEMO_MODE` is a server-only env var; the proxy uses it to
diff --git a/src/components/labs/lab-biomarker-detail.tsx b/src/components/labs/lab-biomarker-detail.tsx
index cb2162230..978153daf 100644
--- a/src/components/labs/lab-biomarker-detail.tsx
+++ b/src/components/labs/lab-biomarker-detail.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useMemo, useState } from "react";
+import { useEffect, useMemo, useState } from "react";
 import Link from "next/link";
 import { useRouter } from "next/navigation";
 import {
@@ -27,7 +27,11 @@ import { apiDelete, apiGet } from "@/lib/api/api-fetch";
 import { summarize, type DataSummary } from "@/lib/analytics/trends";
 import { BIOMARKER_CATALOG } from "@/lib/labs/biomarker-catalog";
 import { useTranslations } from "@/lib/i18n/context";
-import { fallbackMessages } from "@/lib/i18n/load-locale";
+import {
+  getFallbackMessages,
+  loadMessages,
+  type MessageBundle,
+} from "@/lib/i18n/load-locale";
 import { resolveKey } from "@/lib/i18n/resolve-key";
 import { queryKeys } from "@/lib/query-keys";
 
@@ -92,23 +96,40 @@ export function LabBiomarkerDetail({ biomarkerId }: { biomarkerId: string }) {
   // marker minted under the English default and then viewed in another locale
   // never matched a map built only from the current locale's names — the rich
   // description silently fell through to the generic line. The English bundle
-  // is always in the client chunk (`fallbackMessages`), so adding its names
-  // makes resolution independent of the locale the marker was created in.
+  // resolves through `getFallbackMessages()` (already in hand on the server
+  // and for EN sessions; lazily fetched below otherwise — the EN catalog is
+  // no longer a static every-route import), so adding its names makes
+  // resolution independent of the locale the marker was created in.
+  const [enBundle, setEnBundle] = useState<MessageBundle | undefined>(
+    getFallbackMessages,
+  );
+  useEffect(() => {
+    if (enBundle) return;
+    let cancelled = false;
+    void loadMessages("en")
+      .then((loaded) => {
+        if (!cancelled) setEnBundle(loaded);
+      })
+      .catch(() => {
+        // Offline — the current-locale names still resolve; only markers
+        // minted under another locale keep the generic description.
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [enBundle]);
   const slugByName = useMemo(() => {
     const map = new Map<string, string>();
     for (const seed of BIOMARKER_CATALOG) {
       const localized = t(`labs.catalog.${seed.slug}`).trim().toLowerCase();
       if (localized) map.set(localized, seed.slug);
-      const canonical = resolveKey(
-        fallbackMessages,
-        `labs.catalog.${seed.slug}`,
-      )
-        ?.trim()
-        .toLowerCase();
+      const canonical = enBundle
+        ? resolveKey(enBundle, `labs.catalog.${seed.slug}`)?.trim().toLowerCase()
+        : undefined;
       if (canonical && !map.has(canonical)) map.set(canonical, seed.slug);
     }
     return map;
-  }, [t]);
+  }, [t, enBundle]);
   const [addOpen, setAddOpen] = useState(false);
   // Sticky-footer slot for the add-value sheet (the form portals here).
   const [addFooterEl, setAddFooterEl] = useState<HTMLDivElement | null>(null);
diff --git a/src/components/providers.tsx b/src/components/providers.tsx
index b6a97b360..10ec7fae9 100644
--- a/src/components/providers.tsx
+++ b/src/components/providers.tsx
@@ -182,12 +182,9 @@ function QueryPersistenceBridge() {
 export function Providers({
   children,
   initialLocale,
-  initialMessages,
 }: {
   children: ReactNode;
   initialLocale?: Locale;
-  /** Active locale's bundle, server-resolved by the root layout (RSC handoff). */
-  initialMessages?: Record<string, unknown>;
 }) {
   const [queryClient] = useState(
     () =>
@@ -206,10 +203,7 @@ export function Providers({
   return (
     <QueryClientProvider client={queryClient}>
       <ThemeProvider>
-        <I18nProvider
-          initialLocale={initialLocale}
-          initialMessages={initialMessages}
-        >
+        <I18nProvider initialLocale={initialLocale}>
           <QueryPersistenceBridge />
           <DashboardSnapshotPreloader />
           {children}
diff --git a/src/lib/i18n/context.tsx b/src/lib/i18n/context.tsx
index 2add8a6d1..3ce60717f 100644
--- a/src/lib/i18n/context.tsx
+++ b/src/lib/i18n/context.tsx
@@ -22,8 +22,8 @@ import { readStoredTimeFormat, subscribeTimeFormat } from "../time-format";
 import { readStoredDateFormat, subscribeDateFormat } from "../date-format";
 import { resolveKey } from "./resolve-key";
 import {
-  fallbackMessages,
   getCachedMessages,
+  getFallbackMessages,
   loadMessages,
   primeMessages,
   type MessageBundle,
@@ -117,12 +117,13 @@ export function I18nProvider({
   children: ReactNode;
   initialLocale?: Locale;
   /**
-   * The active locale's message bundle, resolved server-side by the
-   * root layout and passed through the RSC payload. This is what keeps
-   * the client chunk free of every non-EN bundle WITHOUT reintroducing
-   * the EN→DE hydration flash: the first client render already holds
-   * the right strings, no async fetch on first paint. Only EN ships
-   * statically (the synchronous fallback floor of the t() chain).
+   * Optional pre-resolved bundle for standalone mounts (tests, embedded
+   * providers). The APP no longer passes this: the active bundle reaches
+   * the provider through the `load-locale` cache — seeded server-side from
+   * the full catalog map during SSR and client-side by the layout's
+   * versioned `/i18n/<locale>` boot script — so the catalog is never
+   * serialized into the RSC flight payload (it used to be 392 KB of every
+   * dashboard document).
    */
   initialMessages?: MessageBundle;
 }) {
@@ -146,10 +147,25 @@ export function I18nProvider({
       primeMessages(locale, initialMessages);
       return { locale, messages: initialMessages };
     }
-    return { locale, messages: getCachedMessages(locale) ?? fallbackMessages };
+    // Cache miss on every ladder rung (boot script failed AND the locale
+    // isn't EN-cached): render with an empty bundle for one frame — the
+    // backfill effect below dynamic-imports the bundle and flips state.
+    // In the app this only happens when the boot script is unreachable
+    // (evicted HTTP cache offline); keys render raw briefly rather than
+    // in the wrong language.
+    return {
+      locale,
+      messages: getCachedMessages(locale) ?? getFallbackMessages() ?? {},
+    };
   });
   const { locale, messages } = active;
   const [pendingLocale, setPendingLocale] = useState<Locale | null>(null);
+  // Bumped when the lazily-loaded EN fallback bundle lands so every t()
+  // consumer re-renders with the fallback floor in place. The EN catalog is
+  // no longer a static import (it cost ~106 KB gz on every route); it loads
+  // on demand the first time a key actually misses the active bundle.
+  const [, setFallbackTick] = useState(0);
+  const fallbackRequestedRef = useRef(false);
   // Tracks the most recent switch request so a slow bundle load can't
   // clobber a newer one (last click wins).
   const requestedLocaleRef = useRef<Locale | null>(null);
@@ -228,7 +244,22 @@ export function I18nProvider({
 
       // Fallback to English if key missing in current locale
       if (value === undefined && locale !== "en") {
-        value = resolveKey(fallbackMessages, key);
+        const fallback = getFallbackMessages();
+        if (fallback) {
+          value = resolveKey(fallback, key);
+        } else if (!fallbackRequestedRef.current) {
+          // First miss without the EN floor in hand: fetch it once, then
+          // re-render so this key (and any sibling misses) resolve. The
+          // locale-integrity guard keeps every locale key-complete, so in
+          // practice this path only runs against a drifted cached bundle.
+          fallbackRequestedRef.current = true;
+          void loadMessages("en")
+            .then(() => setFallbackTick((n) => n + 1))
+            .catch(() => {
+              // Leave the ref set — retrying every render would hammer a
+              // dead network; the raw key remains the last resort.
+            });
+        }
       }
 
       // Fallback to key itself
diff --git a/src/lib/i18n/load-locale.ts b/src/lib/i18n/load-locale.ts
index 359dcea3e..ef948640a 100644
--- a/src/lib/i18n/load-locale.ts
+++ b/src/lib/i18n/load-locale.ts
@@ -1,33 +1,44 @@
 /**
  * Client-side message-bundle loading.
  *
- * Only English ships statically in the client chunk — it is the
- * synchronous fallback floor of the t() chain (active locale → EN →
- * raw key) and the bundle the provider can always resolve against
- * without an async gap. Every other locale reaches the client one of
- * two ways:
+ * NO catalog ships statically in the client chunk — not even English.
+ * The static EN import that used to live here was the single largest
+ * every-route client cost (334 KB raw / ~106 KB gz on every page load,
+ * for every user, in every language). The t() chain (active locale →
+ * EN → raw key) now resolves its bundles through three paths:
  *
- *   1. First paint: the root layout (a server component) resolves the
- *      active locale from the `healthlog-locale` cookie / the
- *      Accept-Language header and threads that locale's messages into
- *      `I18nProvider` as a serialized RSC prop — so the first client
- *      render already holds the right bundle and there is no EN→DE
- *      hydration flash (the historical reason all six bundles used to
- *      ship statically).
- *   2. Locale switch: `loadMessages()` dynamic-imports the target
- *      bundle on demand; each locale becomes its own async chunk that
- *      only the users who switch to it ever download.
+ *   1. SSR pass: this module seeds its cache with the full server-side
+ *      catalog map (`shared-resolve`) inside a `typeof window ===
+ *      "undefined"` branch. Turbopack compiles the branch away for the
+ *      browser bundle (verified — zero catalog bytes reach any client
+ *      chunk), while the server render of every client component
+ *      resolves the active locale synchronously — SSR text stays
+ *      correct with nothing serialized into the flight payload (the
+ *      previous RSC-prop handoff inlined the whole active catalog into
+ *      EVERY document: 392 KB of a 505 KB dashboard HTML).
+ *   2. First client paint: the root layout emits a deferred, versioned,
+ *      immutable-cacheable `<script src="/i18n/<locale>?v=…">` that
+ *      assigns `self.__HL_I18N` before Next's hydration scripts run.
+ *      The initializer below reads it — the first client render holds
+ *      the same bundle the server rendered with (no EN→DE hydration
+ *      flash), and repeat visits serve the catalog from HTTP/SW cache
+ *      instead of re-downloading it inside each document.
+ *   3. Locale switch / EN fallback: `loadMessages()` dynamic-imports
+ *      the target bundle on demand; each locale becomes its own async
+ *      chunk that only the users who need it ever download.
  *
- * The module-level cache makes repeat switches instant and lets the
- * provider seed it with the server-passed bundle (`primeMessages`).
+ * The module-level cache makes repeat switches instant and lets tests
+ * seed bundles directly (`primeMessages`).
  */
-import enMessages from "../../../messages/en.json";
 import { type Locale } from "./config";
 
 export type MessageBundle = Record<string, unknown>;
 
-/** Synchronous fallback floor — always available, never awaited. */
-export const fallbackMessages: MessageBundle = enMessages;
+/** Shape of the boot global the /i18n/<locale> script assigns. */
+interface I18nBootGlobal {
+  locale?: string;
+  messages?: MessageBundle;
+}
 
 const loaders: Record<Locale, () => Promise<{ default: MessageBundle }>> = {
   de: () => import("../../../messages/de.json"),
@@ -38,13 +49,55 @@ const loaders: Record<Locale, () => Promise<{ default: MessageBundle }>> = {
   pl: () => import("../../../messages/pl.json"),
 };
 
-const cache: Partial<Record<Locale, MessageBundle>> = { en: enMessages };
+function initialCache(): Partial<Record<Locale, MessageBundle>> {
+  if (typeof window === "undefined") {
+    // Server pass (SSR of client components): seed every locale from the
+    // server-side catalog map. The branch is dead code in the browser
+    // bundle — Turbopack eliminates it together with the require, so none
+    // of the six catalogs reaches a client chunk. The try/catch covers
+    // non-bundled node runtimes (vitest transforms this module to ESM,
+    // where a bare require of a TS module throws) — there the cache starts
+    // empty and `vitest.setup.ts` primes every bundle up front.
+    try {
+      // eslint-disable-next-line @typescript-eslint/no-require-imports
+      const { allMessages } =
+        require("./shared-resolve") as typeof import("./shared-resolve");
+      return { ...allMessages };
+    } catch {
+      return {};
+    }
+  }
+  // Browser: the deferred boot script (emitted by the root layout ahead of
+  // the hydration scripts) has already assigned the active locale's bundle.
+  const boot = (self as { __HL_I18N?: I18nBootGlobal }).__HL_I18N;
+  if (
+    boot &&
+    typeof boot.locale === "string" &&
+    boot.messages &&
+    typeof boot.messages === "object"
+  ) {
+    return { [boot.locale as Locale]: boot.messages };
+  }
+  return {};
+}
+
+const cache: Partial<Record<Locale, MessageBundle>> = initialCache();
 
 export function getCachedMessages(locale: Locale): MessageBundle | undefined {
   return cache[locale];
 }
 
-/** Seed the cache with a bundle that arrived as a server prop. */
+/**
+ * The EN fallback floor of the t() chain, when it is already in hand —
+ * always on the server, on the client once EN is the active/boot locale or
+ * `loadMessages("en")` has resolved. `undefined` means "not loaded yet";
+ * callers kick off the async load and re-render when it lands.
+ */
+export function getFallbackMessages(): MessageBundle | undefined {
+  return cache.en;
+}
+
+/** Seed the cache with a bundle that arrived out of band (tests, boot). */
 export function primeMessages(locale: Locale, messages: MessageBundle): void {
   cache[locale] = messages;
 }
diff --git a/src/proxy.ts b/src/proxy.ts
index 0e51bc8e3..6123a9010 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -54,6 +54,10 @@ const PUBLIC_PATHS = [
   // and is not an enumeration oracle — so it must reach the page without
   // an auth gate, like the `/auth/` register surface it forwards to.
   "/invite/",
+  // Locale-catalog boot script (`/i18n/<locale>?v=…`). The login page loads
+  // it pre-auth, and the body is the same public catalog JSON that ships in
+  // the repository — no tenant data, no secrets. Immutable-cacheable.
+  "/i18n/",
   // `/onboarding` itself + its subroutes are matched exactly via
   // `isPublicPath()` so we don't admit `/onboarding-export` etc.
   "/robots.txt",