Hi,
After you activate that setting in the Joomla configuration, you'll have language debug information at the bottom of each page on your website. And it will say which language file were loaded, which one failed, and when it failed, on which line of the file it failed.
So after activating that setting, you need to look for the file mentioned in the alert popup in the debug information. It should tell you on which line it has a problem.
If you could give the lines of which Joomla's debug reported an issue on that file and a copy of that file, we should be able to tell you what to do and maybe we'll be able to understand why this happened.