web: refactor and deduplicate locale preference logic

web/src/lib/i18n/locale.ts

@@ -0,0 +1,31 @@
+import { derived } from 'svelte/store';
+
+import languages from '$i18n/languages.json';
+
+import settings from '$lib/state/settings';
+import { device } from '$lib/device';
+import { INTERNAL_locale, defaultLocale } from '$lib/i18n/translations';
+
+const isValid = (lang: string) => (
+    Object.keys(languages).includes(lang)
+);
+
+export default derived(
+    settings,
+    ($settings) => {
+        let currentLocale = defaultLocale;
+
+        if ($settings.appearance.autoLanguage) {
+            if (isValid(device.prefers.language)) {
+                currentLocale = device.prefers.language;
+            }
+        } else {
+            if (isValid($settings.appearance.language)) {
+                currentLocale = $settings.appearance.language;
+            }
+        }
+
+        INTERNAL_locale.set(currentLocale);
+        return currentLocale;
+    }
+);