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

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);
+}