Do a backup of messages.po and messages.mo.
If you have cpanel, edit messages.po and save it.
Download, open and save with poedit the messages.po file previously modified with code editor in cpanel.
(I'm curious if the error regarding same language will still apear. If yes, then the brutal php file edit is the only way. If not, you are lucky).
If you don't have cpanel, leave your email and I'll contact you.