Improve language search in Plasma Setup
Reason for the change
Closes #51 (closed)
The language selection page in Plasma Setup only searched by locale code and native language name. This meant that users with a US English keyboard layout could not easily find languages by their English names (for example, searching "Russian" or "Japanese" would not return the expected language).
This change adds support for searching languages by their English display name in addition to the existing locale code and native language name matching.
The implementation uses Qt's QLocale API through LanguageUtil instead of maintaining a separate list of language names.
Test plan
Built and ran Plasma Setup locally using kde-builder.
Verified that the language search now works with:
- locale codes (for example: `ru`)
- native language names (for example: `русский`, where supported by installed fonts)
- English language names (for example: `Russian`, `French`, `German`, `Japanese`)
Accent-insensitive matching (for example, searching `Francais` to find `Français`) is not addressed by this change.