diff --git a/src/client/index.tsx b/src/client/index.tsx
index a7af06def..0c8a4edfe 100644
--- a/src/client/index.tsx
+++ b/src/client/index.tsx
@@ -1,16 +1,23 @@
-import { initializeSite, setupDateFns } from "@utils/app";
+import { initializeSite } from "@utils/app";
 import { hydrate } from "inferno-hydrate";
 import { BrowserRouter } from "inferno-router";
 import { App } from "../shared/components/app/app";
+import { loadUserLanguage } from "../shared/services/I18NextService";
+import { verifyDynamicImports } from "../shared/dynamic-imports";
 
 import "bootstrap/js/dist/collapse";
 import "bootstrap/js/dist/dropdown";
 import "bootstrap/js/dist/modal";
 
 async function startClient() {
+  // Allows to test imports from the browser console.
+  window.checkLazyScripts = () => {
+    verifyDynamicImports(true).then(x => console.log(x));
+  };
+
   initializeSite(window.isoData.site_res);
 
-  await setupDateFns();
+  await loadUserLanguage();
 
   const wrapper = (
     <BrowserRouter>
@@ -22,6 +29,8 @@ async function startClient() {
 
   if (root) {
     hydrate(wrapper, root);
+
+    root.dispatchEvent(new CustomEvent("lemmy-hydrated", { bubbles: true }));
   }
 }
 
diff --git a/src/server/handlers/catch-all-handler.tsx b/src/server/handlers/catch-all-handler.tsx
index 3466f3cc2..2672f7160 100644
--- a/src/server/handlers/catch-all-handler.tsx
+++ b/src/server/handlers/catch-all-handler.tsx
@@ -20,9 +20,26 @@ import { createSsrHtml } from "../utils/create-ssr-html";
 import { getErrorPageData } from "../utils/get-error-page-data";
 import { setForwardedHeaders } from "../utils/set-forwarded-headers";
 import { getJwtCookie } from "../utils/has-jwt-cookie";
+import {
+  I18NextService,
+  LanguageService,
+  UserService,
+} from "../../shared/services/";
 
 export default async (req: Request, res: Response) => {
   try {
+    const languages: string[] =
+      req.headers["accept-language"]
+        ?.split(",")
+        .map(x => {
+          const [head, tail] = x.split(/;\s*q?\s*=?/); // at ";", remove "q="
+          const q = Number(tail ?? 1); // no q means q=1
+          return { lang: head.trim(), q: Number.isNaN(q) ? 0 : q };
+        })
+        .filter(x => x.lang)
+        .sort((a, b) => b.q - a.q)
+        .map(x => (x.lang === "*" ? "en" : x.lang)) ?? [];
+
     const activeRoute = routes.find(route => matchPath(req.path, route));
 
     const headers = setForwardedHeaders(req.headers);
@@ -60,6 +77,7 @@ export default async (req: Request, res: Response) => {
     if (try_site.state === "success") {
       site = try_site.data;
       initializeSite(site);
+      LanguageService.updateLanguages(languages);
 
       if (path !== "/setup" && !site.site_view.local_site.site_setup) {
         return res.redirect("/setup");
@@ -73,6 +91,16 @@ export default async (req: Request, res: Response) => {
           headers,
         };
 
+        if (process.env.NODE_ENV === "development") {
+          setTimeout(() => {
+            // Intentionally (likely) break things if fetchInitialData tries to
+            // use global state after the first await of an unresolved promise.
+            // This simulates another request entering or leaving this
+            // "success" block.
+            UserService.Instance.myUserInfo = undefined;
+            I18NextService.i18n.changeLanguage("cimode");
+          });
+        }
         routeData = await activeRoute.fetchInitialData(initialFetchReq);
       }
 
@@ -114,9 +142,20 @@ export default async (req: Request, res: Response) => {
       </StaticRouter>
     );
 
+    // Another request could have initialized a new site.
+    initializeSite(site);
+    LanguageService.updateLanguages(languages);
+
     const root = renderToString(wrapper);
 
-    res.send(await createSsrHtml(root, isoData, res.locals.cspNonce));
+    res.send(
+      await createSsrHtml(
+        root,
+        isoData,
+        res.locals.cspNonce,
+        LanguageService.userLanguages,
+      ),
+    );
   } catch (err) {
     // If an error is caught here, the error page couldn't even be rendered
     console.error(err);
diff --git a/src/server/index.tsx b/src/server/index.tsx
index b3955348f..34fc9cccf 100644
--- a/src/server/index.tsx
+++ b/src/server/index.tsx
@@ -13,6 +13,7 @@ import ThemeHandler from "./handlers/theme-handler";
 import ThemesListHandler from "./handlers/themes-list-handler";
 import { setCacheControl, setDefaultCsp } from "./middleware";
 import CodeThemeHandler from "./handlers/code-theme-handler";
+import { verifyDynamicImports } from "../shared/dynamic-imports";
 
 const server = express();
 
@@ -54,6 +55,8 @@ server.get("/css/themelist", ThemesListHandler);
 server.get("/*", CatchAllHandler);
 
 const listener = server.listen(Number(port), hostname, () => {
+  verifyDynamicImports(true);
+
   setupDateFns();
   console.log(
     `Lemmy-ui v${VERSION} started listening on http://${hostname}:${port}`,
diff --git a/src/server/utils/create-ssr-html.tsx b/src/server/utils/create-ssr-html.tsx
index 0958588d3..1655d3c81 100644
--- a/src/server/utils/create-ssr-html.tsx
+++ b/src/server/utils/create-ssr-html.tsx
@@ -7,6 +7,8 @@ import { favIconPngUrl, favIconUrl } from "../../shared/config";
 import { IsoDataOptionalSite } from "../../shared/interfaces";
 import { buildThemeList } from "./build-themes-list";
 import { fetchIconPng } from "./fetch-icon-png";
+import { findTranslationChunkNames } from "../../shared/services/I18NextService";
+import { findDateFnsChunkNames } from "../../shared/utils/app/setup-date-fns";
 
 const customHtmlHeader = process.env["LEMMY_UI_CUSTOM_HTML_HEADER"] || "";
 
@@ -16,6 +18,7 @@ export async function createSsrHtml(
   root: string,
   isoData: IsoDataOptionalSite,
   cspNonce: string,
+  userLanguages: readonly string[],
 ) {
   const site = isoData.site_res;
 
@@ -63,10 +66,20 @@ export async function createSsrHtml(
 
   const helmet = Helmet.renderStatic();
 
+  const lazyScripts = [
+    ...findTranslationChunkNames(userLanguages),
+    ...findDateFnsChunkNames(userLanguages),
+  ]
+    .filter(x => x !== undefined)
+    .map(x => `${getStaticDir()}/js/${x}.client.js`)
+    .map(x => `<link rel="preload" as="script" href="${x}" />`)
+    .join("");
+
   return `
     <!DOCTYPE html>
     <html ${helmet.htmlAttributes.toString()}>
     <head>
+    ${lazyScripts}
     <script nonce="${cspNonce}">window.isoData = ${serialize(isoData)}</script>
   
     <!-- A remote debugging utility for mobile -->
diff --git a/src/shared/components/app/navbar.tsx b/src/shared/components/app/navbar.tsx
index 900a12ec5..390cd718d 100644
--- a/src/shared/components/app/navbar.tsx
+++ b/src/shared/components/app/navbar.tsx
@@ -78,7 +78,6 @@ export class Navbar extends Component<NavbarProps, NavbarState> {
         UnreadCounterService.Instance.unreadApplicationCountSubject.subscribe(
           unreadApplicationCount => this.setState({ unreadApplicationCount }),
         );
-      this.requestNotificationPermission();
 
       document.addEventListener("mouseup", this.handleOutsideMenuClick);
     }
@@ -468,7 +467,7 @@ export class Navbar extends Component<NavbarProps, NavbarState> {
 
   requestNotificationPermission() {
     if (UserService.Instance.myUserInfo) {
-      document.addEventListener("DOMContentLoaded", function () {
+      document.addEventListener("lemmy-hydrated", function () {
         if (!Notification) {
           toast(I18NextService.i18n.t("notifications_error"), "danger");
           return;
diff --git a/src/shared/components/person/settings.tsx b/src/shared/components/person/settings.tsx
index fe12640e1..23ed4c8ac 100644
--- a/src/shared/components/person/settings.tsx
+++ b/src/shared/components/person/settings.tsx
@@ -45,7 +45,11 @@ import {
   RequestState,
   wrapClient,
 } from "../../services/HttpService";
-import { I18NextService, languages } from "../../services/I18NextService";
+import {
+  I18NextService,
+  languages,
+  loadUserLanguage,
+} from "../../services/I18NextService";
 import { setupTippy } from "../../tippy";
 import { toast } from "../../toast";
 import { HtmlTags } from "../common/html-tags";
@@ -335,6 +339,11 @@ export class Settings extends Component<any, SettingsState> {
     }
   }
 
+  componentWillUnmount(): void {
+    // In case `interface_language` change wasn't saved.
+    loadUserLanguage();
+  }
+
   static async fetchInitialData({
     headers,
   }: InitialFetchRequest): Promise<SettingsData> {
@@ -791,7 +800,7 @@ export class Settings extends Component<any, SettingsState> {
                 onChange={linkEvent(this, this.handleInterfaceLangChange)}
                 className="form-select d-inline-block w-auto"
               >
-                <option disabled aria-hidden="true">
+                <option disabled aria-hidden="true" selected>
                   {I18NextService.i18n.t("interface_language")}
                 </option>
                 <option value="browser">
@@ -1451,6 +1460,12 @@ export class Settings extends Component<any, SettingsState> {
     const newLang = event.target.value ?? "browser";
     I18NextService.i18n.changeLanguage(
       newLang === "browser" ? navigator.languages : newLang,
+      () => {
+        // Now the language is loaded, can be synchronous. Let the state update first.
+        window.requestAnimationFrame(() => {
+          i.forceUpdate();
+        });
+      },
     );
 
     i.setState(
@@ -1549,6 +1564,7 @@ export class Settings extends Component<any, SettingsState> {
         });
 
         UserService.Instance.myUserInfo = siteRes.data.my_user;
+        loadUserLanguage();
       }
 
       toast(I18NextService.i18n.t("saved"));
diff --git a/src/shared/dynamic-imports.ts b/src/shared/dynamic-imports.ts
new file mode 100644
index 000000000..546063feb
--- /dev/null
+++ b/src/shared/dynamic-imports.ts
@@ -0,0 +1,49 @@
+import { verifyTranslationImports } from "./services/I18NextService";
+import { verifyDateFnsImports } from "@utils/app/setup-date-fns";
+
+export class ImportReport {
+  error: Array<{ id: string; error: Error | string | undefined }> = [];
+  success: string[] = [];
+}
+
+export type ImportReportCollection = {
+  translation?: ImportReport;
+  "date-fns"?: ImportReport;
+};
+
+function collect(
+  verbose: boolean,
+  kind: keyof ImportReportCollection,
+  collection: ImportReportCollection,
+  report: ImportReport,
+) {
+  collection[kind] = report;
+  if (verbose) {
+    for (const { id, error } of report.error) {
+      console.warn(`${kind} "${id}" failed: ${error}`);
+    }
+    const good = report.success.length;
+    const bad = report.error.length;
+    if (bad) {
+      console.error(`${bad} out of ${bad + good} ${kind} imports failed.`);
+    } else {
+      console.log(`${good} ${kind} imports verified.`);
+    }
+  }
+}
+
+// This verifies that the parameters used for parameterized imports are
+// correct, that the respective chunks are reachable or bundled, and that the
+// returned objects match expectations.
+export async function verifyDynamicImports(
+  verbose: boolean,
+): Promise<ImportReportCollection> {
+  const collection: ImportReportCollection = {};
+  await verifyTranslationImports().then(report =>
+    collect(verbose, "translation", collection, report),
+  );
+  await verifyDateFnsImports().then(report =>
+    collect(verbose, "date-fns", collection, report),
+  );
+  return collection;
+}
diff --git a/src/shared/interfaces.ts b/src/shared/interfaces.ts
index 304e3d9d1..af8df0695 100644
--- a/src/shared/interfaces.ts
+++ b/src/shared/interfaces.ts
@@ -21,6 +21,7 @@ export type IsoDataOptionalSite<T extends RouteData = any> = Partial<
 declare global {
   interface Window {
     isoData: IsoData;
+    checkLazyScripts?: () => void;
   }
 }
 
diff --git a/src/shared/services/I18NextService.ts b/src/shared/services/I18NextService.ts
index a7e8c979c..4de798d36 100644
--- a/src/shared/services/I18NextService.ts
+++ b/src/shared/services/I18NextService.ts
@@ -1,74 +1,117 @@
 import { isBrowser } from "@utils/browser";
-import i18next, { Resource } from "i18next";
+import i18next, { BackendModule, ReadCallback, Resource } from "i18next";
+import { ImportReport } from "../dynamic-imports";
 import { UserService } from "../services";
-import { ar } from "../translations/ar";
-import { bg } from "../translations/bg";
-import { ca } from "../translations/ca";
-import { cs } from "../translations/cs";
-import { da } from "../translations/da";
-import { de } from "../translations/de";
-import { el } from "../translations/el";
 import { en } from "../translations/en";
-import { eo } from "../translations/eo";
-import { es } from "../translations/es";
-import { eu } from "../translations/eu";
-import { fa } from "../translations/fa";
-import { fi } from "../translations/fi";
-import { fr } from "../translations/fr";
-import { ga } from "../translations/ga";
-import { gl } from "../translations/gl";
-import { hr } from "../translations/hr";
-import { id } from "../translations/id";
-import { it } from "../translations/it";
-import { ja } from "../translations/ja";
-import { ko } from "../translations/ko";
-import { nl } from "../translations/nl";
-import { oc } from "../translations/oc";
-import { pl } from "../translations/pl";
-import { pt } from "../translations/pt";
-import { pt_BR } from "../translations/pt_BR";
-import { ru } from "../translations/ru";
-import { sv } from "../translations/sv";
-import { vi } from "../translations/vi";
-import { zh } from "../translations/zh";
-import { zh_Hant } from "../translations/zh_Hant";
-
-export const languages = [
-  { resource: ar, code: "ar", name: "العربية" },
-  { resource: bg, code: "bg", name: "Български" },
-  { resource: ca, code: "ca", name: "Català" },
-  { resource: cs, code: "cs", name: "Česky" },
-  { resource: da, code: "da", name: "Dansk" },
-  { resource: de, code: "de", name: "Deutsch" },
-  { resource: el, code: "el", name: "Ελληνικά" },
-  { resource: en, code: "en", name: "English" },
-  { resource: eo, code: "eo", name: "Esperanto" },
-  { resource: es, code: "es", name: "Español" },
-  { resource: eu, code: "eu", name: "Euskara" },
-  { resource: fa, code: "fa", name: "فارسی" },
-  { resource: fi, code: "fi", name: "Suomi" },
-  { resource: fr, code: "fr", name: "Français" },
-  { resource: ga, code: "ga", name: "Gaeilge" },
-  { resource: gl, code: "gl", name: "Galego" },
-  { resource: hr, code: "hr", name: "Hrvatski" },
-  { resource: id, code: "id", name: "Bahasa Indonesia" },
-  { resource: it, code: "it", name: "Italiano" },
-  { resource: ja, code: "ja", name: "日本語" },
-  { resource: ko, code: "ko", name: "한국어" },
-  { resource: nl, code: "nl", name: "Nederlands" },
-  { resource: oc, code: "oc", name: "Occitan" },
-  { resource: pl, code: "pl", name: "Polski" },
-  { resource: pt, code: "pt", name: "Português" },
-  { resource: pt_BR, code: "pt_BR", name: "Português (Brasil)" },
-  { resource: ru, code: "ru", name: "Русский" },
-  { resource: sv, code: "sv", name: "Svenska" },
-  { resource: vi, code: "vi", name: "Tiếng Việt" },
-  { resource: zh, code: "zh", name: "中文 (简体)" },
-  { resource: zh_Hant, code: "zh-TW", name: "中文 (繁體)" },
+import { setupDateFns } from "@utils/app";
+
+export type TranslationDesc = {
+  resource: string;
+  code: string;
+  name: string;
+  bundled?: boolean;
+};
+
+export const languages: TranslationDesc[] = [
+  { resource: "ar", code: "ar", name: "العربية" },
+  { resource: "bg", code: "bg", name: "Български" },
+  { resource: "ca", code: "ca", name: "Català" },
+  { resource: "cs", code: "cs", name: "Česky" },
+  { resource: "da", code: "da", name: "Dansk" },
+  { resource: "de", code: "de", name: "Deutsch" },
+  { resource: "el", code: "el", name: "Ελληνικά" },
+  { resource: "en", code: "en", name: "English", bundled: true },
+  { resource: "eo", code: "eo", name: "Esperanto" },
+  { resource: "es", code: "es", name: "Español" },
+  { resource: "eu", code: "eu", name: "Euskara" },
+  { resource: "fa", code: "fa", name: "فارسی" },
+  { resource: "fi", code: "fi", name: "Suomi" },
+  { resource: "fr", code: "fr", name: "Français" },
+  { resource: "ga", code: "ga", name: "Gaeilge" },
+  { resource: "gl", code: "gl", name: "Galego" },
+  { resource: "hr", code: "hr", name: "Hrvatski" },
+  { resource: "id", code: "id", name: "Bahasa Indonesia" },
+  { resource: "it", code: "it", name: "Italiano" },
+  { resource: "ja", code: "ja", name: "日本語" },
+  { resource: "ko", code: "ko", name: "한국어" },
+  { resource: "nl", code: "nl", name: "Nederlands" },
+  { resource: "oc", code: "oc", name: "Occitan" },
+  { resource: "pl", code: "pl", name: "Polski" },
+  { resource: "pt", code: "pt", name: "Português" },
+  { resource: "pt_BR", code: "pt-BR", name: "Português (Brasil)" },
+  { resource: "ru", code: "ru", name: "Русский" },
+  { resource: "sv", code: "sv", name: "Svenska" },
+  { resource: "vi", code: "vi", name: "Tiếng Việt" },
+  { resource: "zh", code: "zh", name: "中文 (简体)" },
+  { resource: "zh_Hant", code: "zh-TW", name: "中文 (繁體)" },
 ];
 
-const resources: Resource = {};
-languages.forEach(l => (resources[l.code] = l.resource));
+const languageByCode = languages.reduce((acc, l) => {
+  acc[l.code] = l;
+  return acc;
+}, {});
+
+// Use pt-BR for users with removed interface language pt_BR.
+languageByCode["pt_BR"] = languageByCode["pt-BR"];
+
+async function load(translation: TranslationDesc): Promise<Resource> {
+  const { resource } = translation;
+  return import(
+    /* webpackChunkName: `translation-[request]`  */
+    `../translations/${resource}`
+  ).then(x => x[resource]);
+}
+
+export async function verifyTranslationImports(): Promise<ImportReport> {
+  const report = new ImportReport();
+  const promises = languages.map(lang =>
+    load(lang)
+      .then(x => {
+        if (x && x["translation"]) {
+          report.success.push(lang.code);
+        } else {
+          throw "unexpected format";
+        }
+      })
+      .catch(err => report.error.push({ id: lang.code, error: err })),
+  );
+  await Promise.all(promises);
+  return report;
+}
+
+export function pickTranslations(lang: string): TranslationDesc[] | undefined {
+  const primary = languageByCode[lang];
+  const [head] = (primary?.code ?? lang).split("-");
+  const secondary = head !== lang ? languageByCode[head] : undefined;
+  if (primary && secondary) {
+    return [primary, secondary];
+  } else if (primary) {
+    return [primary];
+  } else if (secondary) {
+    return [secondary];
+  }
+  return undefined;
+}
+
+export function findTranslationChunkNames(
+  languages: readonly string[],
+): string[] {
+  for (const lang of languages) {
+    const translations = pickTranslations(lang);
+    if (!translations) {
+      continue;
+    }
+    return translations
+      .filter(x => !x.bundled)
+      .map(x => `translation-${x.resource}`);
+  }
+  return [];
+}
+
+export async function loadUserLanguage() {
+  await new Promise(r => I18NextService.i18n.changeLanguage(undefined, r));
+  await setupDateFns();
+}
 
 function format(value: any, format: any): any {
   return format === "uppercase" ? value.toUpperCase() : value;
@@ -78,17 +121,53 @@ class LanguageDetector {
   static readonly type = "languageDetector";
 
   detect() {
-    const langs: string[] = [];
+    return LanguageService.userLanguages;
+  }
+}
 
+export class LanguageService {
+  private static _serverLanguages: readonly string[] = [];
+  private static get languages(): readonly string[] {
+    if (isBrowser()) {
+      return navigator.languages;
+    } else {
+      return this._serverLanguages;
+    }
+  }
+  static updateLanguages(languages: readonly string[]) {
+    this._serverLanguages = languages;
+    I18NextService.i18n.changeLanguage();
+    setupDateFns();
+  }
+  static get userLanguages(): readonly string[] {
     const myLang =
       UserService.Instance.myUserInfo?.local_user_view.local_user
         .interface_language ?? "browser";
+    if (myLang === "browser") {
+      return this.languages;
+    }
+    return [myLang, ...this.languages];
+  }
+}
 
-    if (myLang !== "browser") langs.push(myLang);
+class LazyLoader implements Omit<BackendModule, "type"> {
+  static readonly type = "backend";
 
-    if (isBrowser()) langs.push(...navigator.languages);
+  init() {}
 
-    return langs;
+  read(language: string, namespace: string, cb: ReadCallback): void {
+    const translation: TranslationDesc = languageByCode[language];
+    if (!translation) {
+      cb(new Error(`No translation found: ${language} ${namespace}`), false);
+      return;
+    }
+    load(translation)
+      .then(data => {
+        const resKeys = data && data[namespace];
+        if (!resKeys) throw Error(`Failed loading: ${language} ${namespace}`);
+        cb(null, resKeys);
+      })
+      .catch(err => cb(err, false));
   }
 }
 
@@ -98,17 +177,21 @@ export class I18NextService {
 
   private constructor() {
     this.#i18n = i18next;
-    this.#i18n.use(LanguageDetector).init({
-      debug: false,
-      compatibilityJSON: "v3",
-      supportedLngs: languages.map(l => l.code),
-      nonExplicitSupportedLngs: true,
-      // load: 'languageOnly',
-      // initImmediate: false,
-      fallbackLng: "en",
-      resources,
-      interpolation: { format },
-    });
+    this.#i18n
+      .use(LanguageDetector)
+      .use(LazyLoader)
+      .init({
+        debug: false,
+        compatibilityJSON: "v3",
+        supportedLngs: languages.map(l => l.code),
+        nonExplicitSupportedLngs: true,
+        load: "all",
+        // initImmediate: false,
+        fallbackLng: "en",
+        resources: { en } as Resource,
+        interpolation: { format },
+        partialBundledLanguages: true,
+      });
   }
 
   static get #Instance() {
diff --git a/src/shared/services/index.ts b/src/shared/services/index.ts
index 1fe91f98d..543def382 100644
--- a/src/shared/services/index.ts
+++ b/src/shared/services/index.ts
@@ -1,5 +1,5 @@
 export { FirstLoadService } from "./FirstLoadService";
 export { HttpService } from "./HttpService";
-export { I18NextService } from "./I18NextService";
+export { I18NextService, LanguageService } from "./I18NextService";
 export { UserService } from "./UserService";
 export { UnreadCounterService } from "./UnreadCounterService";
diff --git a/src/shared/utils/app/initialize-site.ts b/src/shared/utils/app/initialize-site.ts
index 565067032..b9fa1f099 100644
--- a/src/shared/utils/app/initialize-site.ts
+++ b/src/shared/utils/app/initialize-site.ts
@@ -1,12 +1,11 @@
 import { GetSiteResponse } from "lemmy-js-client";
 import { setupEmojiDataModel, setupMarkdown } from "../../markdown";
-import { I18NextService, UserService } from "../../services";
+import { UserService } from "../../services";
 import { updateDataBsTheme } from "@utils/browser";
 
 export default function initializeSite(site?: GetSiteResponse) {
   UserService.Instance.myUserInfo = site?.my_user;
   updateDataBsTheme(site);
-  I18NextService.i18n.changeLanguage();
   if (site) {
     setupEmojiDataModel(site.custom_emojis ?? []);
   }
diff --git a/src/shared/utils/app/setup-date-fns.ts b/src/shared/utils/app/setup-date-fns.ts
index 5c4f12fb1..8c3c14550 100644
--- a/src/shared/utils/app/setup-date-fns.ts
+++ b/src/shared/utils/app/setup-date-fns.ts
@@ -1,10 +1,121 @@
-import { setDefaultOptions } from "date-fns";
-import { I18NextService } from "../../services";
+import { setDefaultOptions, Locale } from "date-fns";
+import {
+  I18NextService,
+  LanguageService,
+  pickTranslations,
+} from "../../services/I18NextService";
+import { enUS } from "date-fns/locale/en-US";
+import { ImportReport } from "../../dynamic-imports";
+
+type DateFnsDesc = { resource: string; code: string; bundled?: boolean };
+
+const locales: DateFnsDesc[] = [
+  { resource: "af", code: "af" },
+  { resource: "ar", code: "ar" },
+  { resource: "ar-DZ", code: "ar-DZ" },
+  { resource: "ar-EG", code: "ar-EG" },
+  { resource: "ar-MA", code: "ar-MA" },
+  { resource: "ar-SA", code: "ar-SA" },
+  { resource: "ar-TN", code: "ar-TN" },
+  { resource: "az", code: "az" },
+  { resource: "be", code: "be" },
+  { resource: "be-tarask", code: "be-tarask" },
+  { resource: "bg", code: "bg" },
+  { resource: "bn", code: "bn" },
+  { resource: "bs", code: "bs" },
+  { resource: "ca", code: "ca" },
+  { resource: "cs", code: "cs" },
+  { resource: "cy", code: "cy" },
+  { resource: "da", code: "da" },
+  { resource: "de", code: "de" },
+  { resource: "de-AT", code: "de-AT" },
+  { resource: "el", code: "el" },
+  { resource: "en-AU", code: "en-AU" },
+  { resource: "en-CA", code: "en-CA" },
+  { resource: "en-GB", code: "en-GB" },
+  { resource: "en-IE", code: "en-IE" },
+  { resource: "en-IN", code: "en-IN" },
+  { resource: "en-NZ", code: "en-NZ" },
+  { resource: "en-US", code: "en-US", bundled: true },
+  { resource: "en-ZA", code: "en-ZA" },
+  { resource: "eo", code: "eo" },
+  { resource: "es", code: "es" },
+  { resource: "et", code: "et" },
+  { resource: "eu", code: "eu" },
+  { resource: "fa-IR", code: "fa-IR" },
+  { resource: "fi", code: "fi" },
+  { resource: "fr", code: "fr" },
+  { resource: "fr-CA", code: "fr-CA" },
+  { resource: "fr-CH", code: "fr-CH" },
+  { resource: "fy", code: "fy" },
+  { resource: "gd", code: "gd" },
+  { resource: "gl", code: "gl" },
+  { resource: "gu", code: "gu" },
+  { resource: "he", code: "he" },
+  { resource: "hi", code: "hi" },
+  { resource: "hr", code: "hr" },
+  { resource: "ht", code: "ht" },
+  { resource: "hu", code: "hu" },
+  { resource: "hy", code: "hy" },
+  { resource: "id", code: "id" },
+  { resource: "is", code: "is" },
+  { resource: "it", code: "it" },
+  { resource: "it-CH", code: "it-CH" },
+  { resource: "ja", code: "ja" },
+  { resource: "ja-Hira", code: "ja-Hira" },
+  { resource: "ka", code: "ka" },
+  { resource: "kk", code: "kk" },
+  { resource: "km", code: "km" },
+  { resource: "kn", code: "kn" },
+  { resource: "ko", code: "ko" },
+  { resource: "lb", code: "lb" },
+  { resource: "lt", code: "lt" },
+  { resource: "lv", code: "lv" },
+  { resource: "mk", code: "mk" },
+  { resource: "mn", code: "mn" },
+  { resource: "ms", code: "ms" },
+  { resource: "mt", code: "mt" },
+  { resource: "nb", code: "nb" },
+  { resource: "nl", code: "nl" },
+  { resource: "nl-BE", code: "nl-BE" },
+  { resource: "nn", code: "nn" },
+  { resource: "oc", code: "oc" },
+  { resource: "pl", code: "pl" },
+  { resource: "pt", code: "pt" },
+  { resource: "pt-BR", code: "pt-BR" },
+  { resource: "ro", code: "ro" },
+  { resource: "ru", code: "ru" },
+  { resource: "sk", code: "sk" },
+  { resource: "sl", code: "sl" },
+  { resource: "sq", code: "sq" },
+  { resource: "sr", code: "sr" },
+  { resource: "sr-Latn", code: "sr-Latn" },
+  { resource: "sv", code: "sv" },
+  { resource: "ta", code: "ta" },
+  { resource: "te", code: "te" },
+  { resource: "th", code: "th" },
+  { resource: "tr", code: "tr" },
+  { resource: "ug", code: "ug" },
+  { resource: "uk", code: "uk" },
+  { resource: "uz", code: "uz" },
+  { resource: "uz-Cyrl", code: "uz-Cyrl" },
+  { resource: "vi", code: "vi" },
+  { resource: "zh-CN", code: "zh-CN" },
+  { resource: "zh-HK", code: "zh-HK" },
+  { resource: "zh-TW", code: "zh-TW" },
+];
+
+const localeByCode = locales.reduce((acc, l) => {
+  acc[l.code] = l;
+  return acc;
+}, {});
+
+// Use pt-BR for users with removed interface language pt_BR.
+localeByCode["pt_BR"] = localeByCode["pt-BR"];
 
 const EN_US = "en-US";
 
-export default async function () {
-  let lang = I18NextService.i18n.language;
+function langToLocale(lang: string): DateFnsDesc | undefined {
   if (lang === "en") {
     lang = EN_US;
   }
@@ -19,27 +130,75 @@ export default async function () {
     }
   }
 
-  let locale;
+  return localeByCode[lang];
+}
+
+async function load(locale: DateFnsDesc): Promise<Locale> {
+  return import(
+    /* webpackChunkName: `date-fns-[request]` */
+    `date-fns/locale/${locale.resource}.mjs`
+  ).then(x => x.default);
+}
+
+export async function verifyDateFnsImports(): Promise<ImportReport> {
+  const report = new ImportReport();
+  const promises = locales.map(locale =>
+    load(locale)
+      .then(x => {
+        if (x && x.code === locale.code) {
+          report.success.push(locale.code);
+        } else {
+          throw "unexpected format";
+        }
+      })
+      .catch(err => report.error.push({ id: locale.code, error: err })),
+  );
+  await Promise.all(promises);
+  return report;
+}
+
+function bestDateFns(
+  languages: readonly string[],
+  i18n_full_lang: string,
+): DateFnsDesc {
+  const [base_lang] = i18n_full_lang.split("-");
+  for (const lang of languages.filter(x => x.startsWith(base_lang))) {
+    const locale = langToLocale(lang);
+    if (locale) {
+      return locale;
+    }
+  }
+  // Fallback to base langauge first, to avoid mixing languages.
+  return langToLocale(base_lang) ?? localeByCode[EN_US];
+}
+
+export function findDateFnsChunkNames(languages: readonly string[]): string[] {
+  let i18n_full_lang = EN_US;
+  for (const lang of languages) {
+    if (pickTranslations(lang)) {
+      i18n_full_lang = lang;
+      break;
+    }
+  }
+  const locale = bestDateFns(languages, i18n_full_lang);
+  if (locale.bundled) {
+    return [];
+  }
+  return [`date-fns-${locale.resource}-mjs`];
+}
 
+export default async function () {
+  const i18n_full_lang = I18NextService.i18n.resolvedLanguage ?? EN_US;
+  const localeDesc = bestDateFns(LanguageService.userLanguages, i18n_full_lang);
   try {
-    locale = (
-      await import(
-        /* webpackExclude: /\.js\.flow$/ */
-        `date-fns/locale`
-      )
-    )[lang];
-  } catch (e) {
-    console.log(
-      `Could not load locale ${lang} from date-fns, falling back to ${EN_US}`,
-    );
-    locale = (
-      await import(
-        /* webpackExclude: /\.js\.flow$/ */
-        `date-fns/locale`
-      )
-    )[EN_US];
+    const locale = await load(localeDesc);
+    if (locale) {
+      setDefaultOptions({ locale });
+      return;
+    }
+  } catch {
+    console.error(`Loading ${localeDesc.code} date-fns failed.`);
   }
-  setDefaultOptions({
-    locale,
-  });
+
+  setDefaultOptions({ locale: enUS });
 }
diff --git a/webpack.config.js b/webpack.config.js
index d855b4753..7f0fb9ff5 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -63,6 +63,12 @@ module.exports = (env, argv) => {
       new webpack.BannerPlugin({
         banner,
       }),
+      // helps import("date-fns/locale/${x}.mjs") find "date-fns/locale"
+      new webpack.ContextReplacementPlugin(
+        /date-fns\/locale/,
+        resolve(__dirname, "node_modules/date-fns/locale"),
+        false,
+      ),
     ],
   };
 
@@ -73,6 +79,7 @@ module.exports = (env, argv) => {
       ...base.output,
       filename: "js/server.js",
       publicPath: "/",
+      chunkLoading: false, // everything bundled
     },
     target: "node",
     externals: [nodeExternals(), "inferno-helmet"],
@@ -85,6 +92,7 @@ module.exports = (env, argv) => {
       ...base.output,
       filename: "js/client.js",
       publicPath: `/static/${env.COMMIT_HASH}/`,
+      chunkFilename: "js/[name].client.js", // predictable names for manual preload
     },
     plugins: [
       ...base.plugins,