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;
+    }
+);

web/src/lib/i18n/translations.ts

@@ -1,6 +1,6 @@
 import i18n from 'sveltekit-i18n';
 
-import type { Config } from 'sveltekit-i18n'
+import type { Config } from 'sveltekit-i18n';
 import type {
     GenericImport,
     StructuredLocfileInfo,
@@ -45,6 +45,6 @@ const config: Config = {
 
 export { defaultLocale };
 export const {
-    t, loading, locales, locale, translations,
+    t, loading, locales, locale: INTERNAL_locale, translations,
     loadTranslations, addTranslations, setLocale, setRoute
 } = new i18n(config);