It depends on what you mean by useful. English is one of the most widely spoken or used languages in the world. So that could mean that it’s the most useful. It’s also one of the most universal languages. You could say that as an alternative. I hope this helps you. Good luck

Thats not wrong for the first one as it can be used politely, you can also use most. The thing is you can say it both ways but the more polite way would be the first one, but it really matters who your talking to.

I don't think so. If you want to try to say "English is the most useful language in the world." You should say "No language is as useful as English". By the way, I am a learner.