feat(i18n): add Japanese (ja) translation (#682)

- Add frontend/src/lang/translations/ja.json (full translation of all 422 entries from en.json)
- Register ja in frontend/src/lang/index.js so it appears in the language selector

Tested locally with v2.2.3 build. UI strings render correctly in Japanese.

Closes: language support request

Co-authored-by: Taro Kawakami <tarokun@tunagufactory.jp>

frontend/src/lang/index.js

@@ -9,6 +9,7 @@ import uk from './translations/uk.json';
 import vi from './translations/vi.json';
 import zh from './translations/zh.json';
 import es from './translations/es.json';
+import ja from './translations/ja.json';
 
 import {createI18n} from "vue-i18n";
 
@@ -33,7 +34,8 @@ const i18n = createI18n({
     "uk": uk,
     "vi": vi,
     "zh": zh,
-    "es": es,    
+    "es": es,
+    "ja": ja,
   }
 });