feat: Translate en.json in ja [Manual Sync]

96% of minimum 50% translated source file: '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

@@ -42,7 +42,8 @@
     "update": "更新",
     "upload": "アップロード",
     "openFile": "ファイルを開く",
-    "discardChanges": "Discard"
+  "discardChanges": "Discard",
+  "saveChanges": "Save changes"
   },
   "download": {
     "downloadFile": "ファイルのダウンロード",