Clients are not allowed to ask freelancers to work for free.
But it doesn't sound like this client is doing that. The client sounds like he may just want to verify your language ability.
If the client's request aren't too extensive... If they take only a few minutes, I would say it is okay.
But be protective of your time. Don't get tricked by a client who is asking you to take a language test but it's actually a trick to do homework, or take a school test, or do work that the client is selling to other people.
Thank you for the response!
It is a short test, and there is no way to be homework or smth like that. Also, the client spent a lot of money on Upwork. I wanted to be sure it is OK to be tested.
Thank you once again!
He sounds like a serious client.
Scammers typically make it a point to never pay anything. Scammers don't "pay a lot of money" on Upwork. Only real clients do that.