feat(web): use browser language by default (#10849)

web/src/lib/components/user-settings-page/app-settings.svelte

@@ -14,9 +14,10 @@
     sidebarSettings,
   } from '$lib/stores/preferences.store';
   import { findLocale } from '$lib/utils';
+  import { getClosestAvailableLocale, langCodes } from '$lib/utils/i18n';
   import { onMount } from 'svelte';
+  import { locale as i18nLocale, t } from 'svelte-i18n';
   import { fade } from 'svelte/transition';
-  import { t, init } from 'svelte-i18n';
   import { invalidateAll } from '$app/navigation';
 
   let time = new Date();
@@ -37,6 +38,7 @@
     value: findLocale(editedLocale).code || fallbackLocale.code,
     label: findLocale(editedLocale).name || fallbackLocale.name,
   };
+  $: closestLanguage = getClosestAvailableLocale([$lang], langCodes);
 
   onMount(() => {
     const interval = setInterval(() => {
@@ -78,13 +80,7 @@
   const handleLanguageChange = async (newLang: string | undefined) => {
     if (newLang) {
       $lang = newLang;
-
-      if (newLang === 'dev') {
-        // Reload required, because fallbackLocale cannot be cleared.
-        window.location.reload();
-      }
-
-      await init({ fallbackLocale: defaultLang.code, initialLocale: newLang });
+      await i18nLocale.set(newLang);
       await invalidateAll();
     }
   };
@@ -111,7 +107,7 @@
       <div class="ml-4">
         <SettingCombobox
           comboboxPlaceholder={$t('language')}
-          selectedOption={langOptions.find(({ value }) => value === $lang) || defaultLangOption}
+          selectedOption={langOptions.find(({ value }) => value === closestLanguage) || defaultLangOption}
           options={langOptions}
           title={$t('language')}
           subtitle={$t('language_setting_description')}

web/src/lib/i18n.spec.ts

@@ -1,5 +1,6 @@
 import { langs } from '$lib/constants';
 import messages from '$lib/i18n/en.json';
+import { getClosestAvailableLocale } from '$lib/utils/i18n';
 import { exec as execCallback } from 'node:child_process';
 import { readFileSync, readdirSync } from 'node:fs';
 import { promisify } from 'node:util';
@@ -52,4 +53,30 @@ describe('i18n', () => {
       });
     }
   });
+
+  describe('getClosestAvailableLocale', () => {
+    const allLocales = ['ar', 'bg', 'en', 'en-US', 'en-DE', 'zh-Hans', 'zh-Hans-HK'];
+
+    it('returns undefined on mismatch', () => {
+      expect(getClosestAvailableLocale([], allLocales)).toBeUndefined();
+      expect(getClosestAvailableLocale(['invalid'], allLocales)).toBeUndefined();
+    });
+
+    it('returns the first matching locale', () => {
+      expect(getClosestAvailableLocale(['invalid', 'ar', 'bg'], allLocales)).toBe('ar');
+      expect(getClosestAvailableLocale(['bg'], allLocales)).toBe('bg');
+      expect(getClosestAvailableLocale(['bg', 'invalid', 'ar'], allLocales)).toBe('bg');
+    });
+
+    it('returns the locale for a less specific match', () => {
+      expect(getClosestAvailableLocale(['ar-AE'], allLocales)).toBe('ar-AE');
+      expect(getClosestAvailableLocale(['ar-AE', 'en'], allLocales)).toBe('ar-AE');
+      expect(getClosestAvailableLocale(['zh-Hans-HK', 'zh-Hans'], allLocales)).toBe('zh-Hans-HK');
+    });
+
+    it('ignores the locale for a more specific match', () => {
+      expect(getClosestAvailableLocale(['zh'], allLocales)).toBeUndefined();
+      expect(getClosestAvailableLocale(['de', 'zh', 'en-US'], allLocales)).toBe('en-US');
+    });
+  });
 });

web/src/lib/stores/preferences.store.ts

@@ -1,5 +1,6 @@
 import { browser } from '$app/environment';
 import { Theme, defaultLang } from '$lib/constants';
+import { getPreferredLocale } from '$lib/utils/i18n';
 import { persisted } from 'svelte-local-storage-store';
 import { get } from 'svelte/store';
 
@@ -42,7 +43,8 @@ export const locale = persisted<string | undefined>('locale', undefined, {
   },
 });
 
-export const lang = persisted('lang', defaultLang.code, {
+const preferredLocale = browser ? getPreferredLocale() : undefined;
+export const lang = persisted<string>('lang', preferredLocale || defaultLang.code, {
   serializer: {
     parse: (text) => text,
     stringify: (object) => object ?? '',

web/src/lib/utils/i18n.ts

@@ -1,3 +1,4 @@
+import { langs } from '$lib/constants';
 import { locale, t, waitLocale } from 'svelte-i18n';
 import { get, type Unsubscriber } from 'svelte/store';
 
@@ -11,3 +12,22 @@ export async function getFormatter() {
   await waitLocale();
   return get(t);
 }
+
+// https://github.com/kaisermann/svelte-i18n/blob/780932a3e1270d521d348aac8ba03be9df309f04/src/runtime/stores/locale.ts#L11
+function getSubLocales(refLocale: string) {
+  return refLocale
+    .split('-')
+    .map((_, i, arr) => arr.slice(0, i + 1).join('-'))
+    .reverse();
+}
+
+export function getClosestAvailableLocale(locales: readonly string[], allLocales: readonly string[]) {
+  const allLocalesSet = new Set(allLocales);
+  return locales.find((locale) => getSubLocales(locale).some((subLocale) => allLocalesSet.has(subLocale)));
+}
+
+export const langCodes = langs.map((lang) => lang.code);
+
+export function getPreferredLocale() {
+  return getClosestAvailableLocale(navigator.languages, langCodes);
+}