feat: Translate frontend/src/i18n/en.json in ja

94% of minimum 50% translated source file: 'frontend/src/i18n/en.json'
on 'ja'.

Sync of partially translated files: 
untranslated content is included with an empty translation 
or source language content depending on file format

frontend/src/i18n/ja.json

@@ -102,6 +102,7 @@
     "username": "ユーザー名",
     "usernameTaken": "ユーザー名はすでに取得されています",
     "wrongCredentials": "ユーザー名またはパスワードが間違っています",
+    "passwordTooShort": "Password must be at least {min} characters",
     "logout_reasons": {
       "inactivity": "You have been logged out due to inactivity."
     }