The existance of the folder named by the language doesn't matter much, because there is no specific code for the translation in the few lines of the common.rpy.
The translation is, as I said before, into the language_dict.json. If you open this file with a text viewer, you can see if the translation is made in your language; but I think it's not, therefore you can't 'force' the translation.
Even so, if you have no knowledges about programming or operating systems, you cannot do this just by copy some files, it might be necessary to make some research and edit some files. It might not be a mistake (or forgotten) about missing language in the menu, but a resource not developed into the game. If it was my language I might myself check this, but it has to be done by a native speaker, usually the people who do the translation are those who cares about the implementation.
