feat(i18n): fallback to English when translation is missing (#554)

- Cache English dictionary for reuse as fallback
- Merge non-English locales with English base (locale on top)
- Fall back to English if a locale fails to load entirely

Co-authored-by: GitHub Copilot <copilot@github.com>

Author: xrgzs

src/app/i18n.ts

@@ -39,13 +39,36 @@ export type Lang = keyof typeof langs
 export type RawDictionary = typeof en.dict
 export type Dictionary = i18n.Flatten<RawDictionary>
 
-// Fetch and flatten the dictionary
+// English dictionary cache for fallback
+let enDictCache: Dictionary | null = null
+
+const fetchEnDict = async (): Promise<Dictionary> => {
+  if (!enDictCache) {
+    const dict: RawDictionary = (await import("~/lang/en/entry")).dict
+    enDictCache = i18n.flatten(dict)
+  }
+  return enDictCache
+}
+
+// Fetch and flatten the dictionary, with English fallback
 const fetchDictionary = async (locale: Lang): Promise<Dictionary> => {
   try {
     const dict: RawDictionary = (await import(`~/lang/${locale}/entry.ts`)).dict
-    return i18n.flatten(dict) // Flatten dictionary for easier access to keys
+    const flatDict = i18n.flatten(dict)
+
+    // If not English, merge with English as fallback (English keys underneath, locale on top)
+    if (locale !== "en") {
+      const enDict = await fetchEnDict()
+      return { ...enDict, ...flatDict } as Dictionary
+    }
+
+    return flatDict
   } catch (err) {
     console.error(`Error loading dictionary for locale: ${locale}`, err)
+    // Fallback to English if the requested locale fails to load
+    if (locale !== "en") {
+      return await fetchEnDict()
+    }
     throw new Error(`Failed to load dictionary for ${locale}`)
   }
 }