fix: refine Japanese localization

[sakurakun999]

Summary

This PR updates and refines the Japanese localization (_locales/ja/messages.json) by completing missing keys, correcting mistranslations.

Key Changes

1. Added Missing Translations (46 keys):

   • Added accurate Japanese translations for 46 previously missing locale keys (including Smart Speed engine options, Shorts player controls, transcript copying, dislike counter integrations, and new UI options).

2. Translation & Terminology Refinements:

   • Fixed literal and mistranslated UI strings (e.g., depressed → 凹型 (押し込み), ask → 毎回確認, dislike → 低評価).
   • Standardized terminology to align with YouTube's official Japanese interface (e.g., サブスクリプション → 登録チャンネル, トレンド → 急上昇).

Files Changed

• _locales/ja/messages.json (+247 lines, -95 lines)

Verification

• Validated JSON structure and confirmed all 571 keys match _locales/en/messages.json.
• Verified localized UI strings in the extension popup and options pages.

I used AI to generate this pull request, but I didn't use AI for the translation.

[Copilot]

Pull request overview

This PR refines the Japanese locale file for the extension by reformatting existing entries, correcting multiple translations/terminology, and adding translations for previously missing keys to better align with the English locale.

Changes:

• Expanded many entries from one-line JSON objects into multi-line objects and adjusted a large set of Japanese UI strings.
• Added Japanese translations for a large batch of previously missing keys (e.g., transcript copy, Shorts controls, Smart Speed options).
• Standardized several terms to match YouTube’s Japanese UI conventions (e.g., トレンド→急上昇, サブスクリプション→登録チャンネル).

Suppressed comments (1)

_locales/ja/messages.json:1648

• 同様に、「登録チャンネル」 ページ… のように引用符直後に半角スペースが入っています。UI上の余計な空白になるため、スペースを削除して統一してください。

    "message": "「登録チャンネル」 ページの「関連度順」セクションを非表示"
  },
  "subscriptionsListLayout": {
    "message": "「登録チャンネル」 ページをリストレイアウトで表示"
  },

---

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (5)

_locales/ja/messages.json:1033

• playbackSpeedHotkey はショートカット設定画面で再生速度プリセットのホットキー/ショートカットをまとめたセクション見出しとして使われています。現在の訳（「再生速度の一発切り替え」）だと意味が限定的なので、英語（"Playback speed hotkeys"）に合わせてショートカットであることが分かる表現にした方がUIとして正確です。

  "playbackSpeedHotkey": {
    "message": "再生速度の一発切り替え"
  },

_locales/ja/messages.json:1621

• PR説明では「すべての571キーが _locales/en/messages.json と一致」とありますが、現状の ja は "message" エントリ数が 572 で en(571) と一致していません（例: ja に displayDayOfTheWeek が存在し、en には存在しません）。このブロックで displayDayOfTheWeak を追加したことで不足キーは埋まっていますが、キー一致の検証結果（および不要キーの整理）も合わせて更新が必要です。

  "displayDayOfTheWeak": {
    "message": "曜日を表示"
  },

_locales/ja/messages.json:1030

• playbackSpeed はプレイヤー/ミキサーの再生速度スライダー等のラベルとして使われていますが、現在の訳（「再生速度の上下切り替え」）だと機能説明が変わってしまい誤解を招きます。英語も単に "Playback speed" なので、短いラベルに戻すのが適切です。

This issue also appears on line 1031 of the same file.

  "playbackSpeed": {
    "message": "再生速度の上下切り替え"
  },

_locales/ja/messages.json:595

• forcedVolume は設定のスイッチで「音量を強制（固定）する」意味ですが、現在の訳（「音量を設定」）だと通常の音量設定（volume スライダー）と区別がつきにくく、"Force Volume" の意図も落ちています。近くの forcedPlaybackSpeed が「固定」を用いているため、同様の語感に揃えると分かりやすいです。

  "forcedVolume": {
    "message": "音量を設定"
  },

_locales/ja/messages.json:1570

• hideProgressBarPreview の訳が「シークバーのプレビューを非表示に」で文が途中で終わっており、UIラベルとして不自然です。英語は "Hide progress bar preview" なので、末尾の「に」を落として完結した表現にしてください。

  "hideProgressBarPreview": {
    "message": "シークバーのプレビューを非表示に"
  },

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (1)

_locales/ja/messages.json:1621

• displayDayOfTheWeak が同一ファイル内で 2 回定義されています（この後方の定義が前方の定義を上書き）。Chrome の i18n メッセージとしてはキーの重複は避け、どちらか片方を削除してください。

  "displayDayOfTheWeak": {
    "message": "曜日を表示"
  },

[ImprovedTube2]

thank you very much @sakurakun999

[ImprovedTube2]

which ones might still make sense discussing?

• Did you come across difficult / impossible decisions? If so, then,
  ...
  • https://images.google.com and or https://trends.google.com can help a lot too.

(I say this by default to document and appreciate translator's efforts while fresh and remembered)

Greetings to @utuhiro78

[sakurakun999]

Thank you for merging my pull request!

For this translation, I mainly referred to the Japanese interface of YouTube and other standard software UI to ensure the terminology feels natural to Japanese users.

Here are the main challenges I encountered and topics that might be worth discussing in the future:

1. Phrasing for "Enable / Disable"
   In English, commands like "Enable [Feature]" place the verb first, so the action is immediately clear. In Japanese, verbs come at the very end of the sentence, which forces users to read the entire label just to figure out if it enables or disables a setting. Japanese UI design often prefers using just the feature name and letting the toggle state indicate the action. The current literal translations can feel a bit heavy to read, so there is room for further refinement.

2. Distinguishing playback speed settings (playbackSpeed vs playbackSpeedButton)
   This was the most difficult part. A literal translation of "Playback speed" and "Playback speed button" makes both items sound nearly identical in Japanese. To convey the actual behavior, I translated playbackSpeed as "Cycle playback speed up/down" (再生速度の上下切り替え) and playbackSpeedButton as "Direct speed change" (再生速度の一発切り替え). In Japanese, "一発" (ippatsu) is a casual expression meaning to perform an action in a single step. It might also be worth reviewing whether the original English labels could be updated to clarify the difference between cycling through speeds and setting a specific speed directly.

Thanks again!