Behavior:

While in the admin area, metabox titles and description appear in site locale's language, while the rest of the admin page is displayed in the user's locale/language.

I've changed l10ni18n - get_locale() -> get_user_locale() and it solved my problem.
But on my setup I'm not using cmb on the front end (which should still use get_locale() ) so obviously that's not the solution.