You’re mixing up “learning” with “studying.” “Studying” is an action which can be done during a time period. There is no need for a result. “Learning” is an action but with a result. (More knowledge) You can study without learning. I think you mean something related to, “I always have an English class after lunch.” or “I always spend 30 minutes after lunch studying English.” or “I always practice English after lunch.” I hope you learn each day during your studying.

if something happens regulary we must use present simple.!

No, it's incorrect. Instead say, "I always LEARN English after lunch." other examples, "I sometimes play football after work." use the present simple tense instead of the present progressive.

I'm learning English after lunch. - Is it correct?

No. Choose one of them-> 1. I always learn English after lunch. 2. I am learning English after lunch.