When a new questions plugin type is installed in moodle/question/type, it can contain a language subfolder with one or more languages.
In all current Moodle versions, however, this path in only partially found when editing questions.

The path for language strings is found everywhere except for the module name at the top left of the edit question window and its associated HELP button. There, moodle tries to find the name string and the help files in moodle/lang/en_utf8 location...

Joseph