Add concurrent connections i18n keys

Add new i18n keys "concurrent_fragments" and "concurrent_fragments_help" to multiple language JSON files (ar, de, en, es, fr, hi, id, it, ja, pl, pt, ru, tr, zh). These expose a UI option for the number of concurrent connections per download with a help string (default: 1, warns about throttling/temporary blocks). Strings were added in English across locales and may need translation.

ytsage/languages/ja.json

@@ -342,7 +342,9 @@
     "filename_format_help": "利用可能な変数: %(title)s, %(uploader)s, %(upload_date)s, %(resolution)s, %(id)s, %(ext)s。標準のyt-dlp出力テンプレート構文がサポートされています。",
     "tab_general": "一般",
     "tab_format": "フォーマット",
-    "tab_file": "ファイル"
+    "tab_file": "ファイル",
+    "concurrent_fragments": "Concurrent Connections",
+    "concurrent_fragments_help": "Number of connections per download. Higher values bypass throttling but may cause temporary blocks if set too high. Default: 1."
   },
   "main_ui": {
     "url_placeholder": "YouTubeの動画URLまたはプレイリストURLを入力",
@@ -624,4 +626,4 @@
     "update_failed": "❌ 更新が失敗しました：{error}",
     "check_failed": "更新の確認に失敗しました。インターネット接続を確認してください。"
   }
-}
+}