frontend: Auto-set YouTube broadcast language

[cloud2519]

Description

This PR introduces a feature to automatically transmit the broadcast language to the YouTube API when creating a broadcast via the YouTube account integration in OBS Studio.
Changes:

• Added a checkSendLanguage checkbox in the YouTube broadcast creation dialog.
• Implemented logic to extract the ISO 639-1 language code from the current OBS Studio UI locale using QLocale.
• Updated the videos.update API call to include both snippet.defaultLanguage and snippet.defaultAudioLanguage.
• Added persistent storage of the checkbox state in the OBS configuration.

Motivation and Context

When using the YouTube account integration to start a broadcast, OBS Studio currently does not send any language metadata. Consequently, YouTube defaults the broadcast language to English. This is problematic for non-English streamers because YouTube's automatic live captioning system often fails or defaults to English, requiring manual intervention for every stream. This PR ensures that accessibility features like auto-captions work correctly from the start.

How Has This Been Tested?

• Added my own YouTube Data API v3 credentials (Client ID and Secret) to CMakePresets.json.
• Built custom OBS locally.
• Authenticated with a YouTube test account.
• Verified that checking "Send OBS Language" correctly populates the language on the YouTube Studio dashboard upon stream creation.
• OS: Windows 11

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Fenrirthviti]

Please properly fill out the PR template instead of replacing it.

[cloud2519]

I have updated the PR description with the properly filled-out template and adjusted the PR title to follow the commit guidelines.
Also, I've fixed the clang-format CI issue, and all checks have now successfully passed!
Please let me know if there's anything else needed. Thanks!