migrate to typescript

src/locales/index.ts

@@ -2,10 +2,18 @@ import en from './en.json';
 import vi from './vi.json';
 import zhHans from './zh_Hans.json';
 
-export const defaultLanguage = 'en';
+interface LanguageInfo {
+    name: string,
+    displayName: string,
+    alternativeLanguageTag: string,
+    aliases?: string[],
+    content: object
+}
+
+export const defaultLanguage: string = 'en';
 
 // To add new languages, please refer to https://ezbookkeeping.mayswind.net/translating
-export const allLanguages = {
+export const allLanguages: Record<string, LanguageInfo> = {
     'en': {
         name: 'English',
         displayName: 'English',