Should I tell my boss the truth about the new guys "bilingualism"?

I think you need to mind your own business and leave it alone. If the guy lied about it, eventually he'll be found out. Your helping him with the language will only enable his lie. Do you really want to support someone who conned his way into the job and keeps on trying to fool people? Do you really want to keep on having to cover for this guy's inadequacies? When your boss does find out the truth, it won't look good for you. Let this guy sink or swim on his own.