Localize Debug Settings comments per viewer language

Summary

The Debug Settings panel (Advanced → Show Debug Settings) shows a help comment
for each setting. Today these comments are always in English, regardless of
the viewer language, because they come from app_settings/settings.xml at runtime.
This makes advanced configuration harder for non-English speakers.

Proposed solution

Load optional per-locale comment overrides from
skins/default/xui/{locale}/settings_comments.xml after the session language is
initialized, and apply them to existing controls in both Global and
PerAccount settings groups via LLControlVariable::setComment().

Fallback (required):

• No file for the locale → keep English comments (current behavior).
• Missing key in the file → keep English for that setting only.

This allows incremental translation without breaking untranslated locales.

Scope of initial implementation

• C++ loader in LLAppViewer::loadLocalizedSettingsComments() (llappviewer.cpp / .h).
• First complete translation: PT-BR
  • skins/default/xui/pt/settings_comments.xml (1,507 settings)
  • Complete remaining labels in skins/default/xui/pt/floater_settings_debug.xml

Rationale

• Improves accessibility for Portuguese-speaking residents and maintainers.
• Pattern is extensible to other locales without changing core settings XML.
• English remains the source of truth; translations are overlays only.

How to verify

1. Build and run the viewer.
2. Set language to Português (Brasil) and restart if needed.
3. Enable Preferences → Advanced → Show Advanced Menu.
4. Open Advanced → Show Debug Settings.
5. Confirm setting help text appears in Portuguese.
6. Switch to a locale without settings_comments.xml → comments stay in English.

Start with LLAppViewer::loadLocalizedSettingsComments() in llappviewer.cpp and its declaration in llappviewer.h, then inspect skins/default/xui/pt/settings_comments.xml and floater_settings_debug.xml. Verify the Portuguese Debug Settings comments and remaining labels, while confirming locales without an override file and settings with missing keys retain English text.