feat: add i18next and react-i18next for internationalization support

frontend/package.json

@@ -13,9 +13,12 @@
   },
   "dependencies": {
     "dompurify": "^3.4.1",
+    "i18next": "^23.16.8",
+    "i18next-browser-languagedetector": "^8.0.4",
     "katex": "^0.16.45",
     "react": "^19.2.5",
     "react-dom": "^19.2.5",
+    "react-i18next": "^15.4.1",
     "react-router-dom": "^7.14.2"
   },
   "devDependencies": {