feat(translator): add custom target language input

- Replace Select with Input for custom language entry
- Users can now type any target language they want
- Add i18n translations for all 8 languages

messages/zh-CN.json

@@ -424,7 +424,8 @@
       "success": "文本对已添加到文件夹",
       "error": "添加文本对到文件夹失败"
     },
-    "autoSave": "自动保存"
+    "autoSave": "自动保存",
+    "customLanguage": "或输入语言..."
   },
   "dictionary": {
     "title": "词典",