Support translations of custom plugins

Issue type

Other type of issue

Description with steps to reproduce

Plugin-specific translation files do not appear to be loaded as described here.
I'm guessing that it's possible (likely?) that this is changed / not implemented in 4.x, but it would be nice to know for sure what the status is. Minimal sample plugin attached.

Supporting files, videos and screenshots

HelloWorld.zip

In which versions of MuseScore Studio is this issue present?

4.6.3

Regression

Yes, this used to work in MuseScore 3.x and now is broken

Operating system

macOS 26.01

Additional context

The note_names plugin in the distribution suffers from this problem - most of the note names are translated because they seem to be built-in to the core Musescore, but the additional note names defined in the plugin-specific translation files are ignored (these can be deleted and the behaviour of the plugin does not change). I'm told that this worked under 3.x hence the regression comment above (my example is not tested under 3.x).

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

Start with the attached HelloWorld.zip minimal plugin and the plugin i18n documentation linked in the report, then compare translation loading in MuseScore 4.6.3 with the note_names plugin, which also reproduces the problem. The work is done when plugin-specific translation files are loaded again and the additional note names are translated as expected.