Resolve ordered browser languages when no profile language is saved

Match all browser preferences using existing locale identifiers and explicit standard-to-legacy aliases, including Veps, Venetian, Latin American Spanish and Uzbek scripts. Prefer supported saved profile choices and fall back to English. Respond to browser languagechange without writing a profile choice. Checks cover all 245 locales and alias/error cases. Browser regression added and syntax-checked; execution pending.

Thanks to xet7 !

client/lib/i18n.js

@@ -2,6 +2,7 @@ import { Meteor } from 'meteor/meteor';
 import { Tracker } from 'meteor/tracker';
 import { ReactiveCache } from '/imports/reactiveCache';
 import { TAPi18n } from '/imports/i18n';
+const { preferredLanguage } = require('/imports/i18n/browserLanguage');
 
 // We save the user language preference in the user profile, and use that to set
 // the language reactively. If the user is not connected we use the language
@@ -37,30 +38,15 @@ Meteor.startup(() => {
   // A stored login token can begin resuming after startup has run. Keep this
   // reactive so the persisted profile language wins when that user arrives,
   // rather than leaving a resumed session in the browser's language.
-  Tracker.autorun(() => {
+  const applyPreferredLanguage = () => {
     const currentUser = ReactiveCache.getCurrentUser();
-    const [language] = [
-      currentUser?.profile?.language,
-      navigator.languages?.at(0),
-      navigator.language,
-      navigator.userLanguage,
-    ].filter(Boolean);
-    if (!language) return;
-
-    // Match the browser tag, then progressively strip trailing subtags until a supported
-    // language is found: e.g. 'zh-Hans-CN' -> 'zh-Hans', 'zh-Hant-TW' -> 'zh-Hant',
-    // 'ja-JP' -> 'ja-JP', bare 'zh' -> 'zh-Hans' (via alias). isLanguageSupported is
-    // case-insensitive, so 'zh-hant' / 'JA-JP' etc. also match.
-    let candidate = language;
-    while (candidate) {
-      if (TAPi18n.isLanguageSupported(candidate)) {
-        Promise.resolve(TAPi18n.setLanguage(candidate)).catch(error => {
-          console.error(`Could not load language ${candidate}:`, error);
-        });
-        break;
-      }
-      const cut = candidate.lastIndexOf('-');
-      candidate = cut > 0 ? candidate.slice(0, cut) : '';
-    }
-  });
+    const language = preferredLanguage(currentUser?.profile?.language, navigator,
+      candidate => TAPi18n.resolveTag(candidate));
+    Promise.resolve(TAPi18n.setLanguage(language)).catch(error => {
+      console.error(`Could not load language ${language}:`, error);
+    });
+  };
+  Tracker.autorun(applyPreferredLanguage);
+  // Browsers can change their ordered preferences without a page reload.
+  window.addEventListener('languagechange', applyPreferredLanguage);
 });

imports/i18n/browserLanguage.js

@@ -0,0 +1,33 @@
+// Browser BCP 47 tags mapped to WeKan's historical locale identifiers.
+const aliases = {
+  vep: 've-PP', vec: 've-CC', vls: 'vl-SS', war: 'wa-RR',
+  'es-419': 'es-LA', 'be-by': 'be-BE', 'ca-valencia': 'ca-valencia',
+  'uz-arab': 'uz-AR', 'uz-latn': 'uz-LA', iw: 'he', ji: 'yi', in: 'id',
+  no: 'nb',
+};
+function matchBrowserLanguage(value, resolveTag) {
+  if (typeof value !== 'string' || !value.trim()) return undefined;
+  const original = resolveTag(value.trim());
+  if (original) return original;
+  let candidate = value.trim().replace(/_/g, '-');
+  while (candidate) {
+    const exact = resolveTag(candidate);
+    if (exact) return exact;
+    const mapped = aliases[candidate.toLowerCase()];
+    if (mapped && resolveTag(mapped)) return resolveTag(mapped);
+    const cut = candidate.lastIndexOf('-');
+    candidate = cut > 0 ? candidate.slice(0, cut) : '';
+  }
+  return undefined;
+}
+function preferredLanguage(profileLanguage, navigatorInfo, resolveTag) {
+  const saved = matchBrowserLanguage(profileLanguage, resolveTag);
+  if (saved) return saved;
+  const browserLanguages = Array.isArray(navigatorInfo.languages) ? navigatorInfo.languages : [];
+  for (const language of [...browserLanguages, navigatorInfo.language, navigatorInfo.userLanguage]) {
+    const supported = matchBrowserLanguage(language, resolveTag);
+    if (supported) return supported;
+  }
+  return resolveTag('en') || 'en';
+}
+module.exports = { matchBrowserLanguage, preferredLanguage };

tests/browserLanguageFallback.test.cjs

@@ -0,0 +1,26 @@
+'use strict';
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const { parseLanguageMetadata } = require('./lib/languageRegistrySource.cjs');
+const { matchBrowserLanguage, preferredLanguage } = require('../imports/i18n/browserLanguage');
+const rows = parseLanguageMetadata(fs.readFileSync(path.join(__dirname, '../imports/i18n/languages.js'), 'utf8'));
+const normalize = tag => tag.toLowerCase().replace(/_/g, '-');
+const resolve = value => rows.find(row => row[2] === value)?.[2] || rows.find(row => normalize(row[2]) === normalize(value))?.[2];
+for (const row of rows) {
+  assert.equal(matchBrowserLanguage(row[2], resolve), row[2]);
+  const browserTag = row[2].replace(/_/g, '-');
+  assert.equal(normalize(matchBrowserLanguage(browserTag.toLowerCase(), resolve)), normalize(row[2]));
+}
+for (const [browser, expected] of Object.entries({vep:'ve-PP', vec:'ve-CC', vls:'vl-SS', war:'wa-RR', 'es-419':'es-LA', 'be-BY':'be-BE', 'uz-Arab-UZ':'uz-AR', 'uz-Latn-UZ':'uz-LA', 'zh-Hant-TW':'zh-Hant', 'fr-CA-u-ca-gregory':'fr-CA', 'en-AU':'en_AU', iw:'he', in:'id', no:'nb'})) {
+  assert.equal(matchBrowserLanguage(browser, resolve), expected, browser);
+}
+assert.equal(preferredLanguage(undefined, {languages:['zz-ZZ','fr-CA','de']}, resolve), 'fr-CA');
+assert.equal(preferredLanguage('', {language:'es-CO'}, resolve), 'es-CO');
+assert.equal(preferredLanguage('de', {languages:['fr-CA']}, resolve), 'de');
+assert.equal(preferredLanguage('unsupported', {languages:['fr-CA']}, resolve), 'fr-CA');
+assert.equal(preferredLanguage(undefined, {languages:['unsupported']}, resolve), 'en');
+assert.equal(preferredLanguage(undefined, {userLanguage:'fi-FI'}, resolve), 'fi');
+assert.equal(matchBrowserLanguage(null, resolve), undefined);
+assert.notEqual(matchBrowserLanguage('vep', resolve), 've');
+console.log(`browserLanguageFallback: ${rows.length} registry locales, standard aliases, ordered preferences and saved-profile precedence passed`);

tests/playwright/specs/50-language-popup.e2e.js

@@ -20,3 +20,18 @@ test('language popup lists every locale and orders country before language flags
     await expect(entry.locator('.language-flags')).toHaveAttribute('dir','ltr');
   }
 });
+
+test('missing profile language follows browser preferences; saved choice wins', async ({ boardPage, user }) => {
+  const db = require('../helpers/db');
+  db.updateOne('users', {_id:user._id}, {$unset:{'profile.language':''}});
+  await boardPage.addInitScript(() => {
+    Object.defineProperty(navigator, 'languages', {configurable:true, get:() => ['zz-ZZ', 'vep']});
+    Object.defineProperty(navigator, 'language', {configurable:true, get:() => 'zz-ZZ'});
+  });
+  await boardPage.reload();
+  await expect(boardPage.locator('html')).toHaveAttribute('lang','ve-PP');
+  expect(db.findOne('users',{_id:user._id}).profile?.language).toBeUndefined();
+  db.updateOne('users', {_id:user._id}, {$set:{'profile.language':'de'}});
+  await boardPage.reload();
+  await expect(boardPage.locator('html')).toHaveAttribute('lang','de');
+});