Improve language discovery, translated search, and Chinese shortcuts

[devmobasa]

Make the language setting easier to find from either interface language, and let Chinese command searches work while preserving existing English results.

• Add Interface Language… to the command palette and tray menu, opening the configurator’s language setting directly.
• Match native language names in configurator search.
• Search translated command labels, descriptions and categories, and refresh cached results when the language changes.
• Keep End, Menu and Delete literal when recording and saving shortcuts. Previously, Chinese recording could save Ctrl+终点 instead of Ctrl+End.
• Give the service button its own Start Service / 启动服务 label and correct the existing setup instructions.
• Update the language-contribution checklist and add regression coverage.

Validation passed in a clean checkout of d3f583e1:

• ./tools/lint-and-test.sh
• ./tools/test-gtk-widgets.sh