Add option to load external GUI language (test translation)

[bovirus]

@plaintool

Please consider if you can to add an option to load an external language file (useful to test new/revised translation).

I belieeve that can help translator to check/revise translation before to upload it.

[plaintool]

Thank you for the suggestion.

I will evaluate the possibility to add support for loading an external language file for testing translations. It seems like a useful feature for translators to validate and adjust strings before submitting updates.

I will check how well it fits into the current localization system and consider it for a future release.

Regards,

[plaintool]

Added option in the menu to load external GUI language from a PO file for testing translations.
Language code is extracted from the filename (e.g. trayslate.be.po → be).
If loading fails, fallback language is en, then system language is used.