In some companies, this is expressly forbidden - it's not permitted for an employee ask a contractor or fellow employee do "personal" work for them, paid or otherwise.
The problem with "one-time favors" like this is that they're rarely one time - once this employee knows that you're willing to do this work for him, at no charge, he'll come back for more. And more.
If, as you say, this individual is proficient in the language(s) involved here, there's really no reason for him to ask you to do this and it would seem he's trying to take advantage of you. He may see your time as less valuable than his own.
If this individual were a stranger who walked up to you on the street asking for this translation, would you do this it for them, for free?