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

95% 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

@@ -101,7 +101,10 @@
     "submit": "ログイン",
     "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"
+    "wrongCredentials": "ユーザー名またはパスワードが間違っています",
+    "logout_reasons": {
+      "inactivity": "You have been logged out due to inactivity."
+    }
   },
   "permanent": "永久",
   "prompts": {