Both are present tense. Learning a language is a special case where you probably want to use "am learning English" if you are currently learning English. Normally, you use these two verb forms to distinguish if you are in the moment performing the action or are speaking in a general way. For example: I ride my bike to school. - This is a general comment about how I transport myself to school. I am riding my bike to school. - At this very moment, you can find me on my bike, riding to school. Notice that this is a verb that requires movement that would be difficult to perform forever. For example, I "am going" to the movies - you won't go to the movies in every moment, but in this moment you are. Certain verbs, like studying or learning, are generally used in the form "am studying" or "am learning." When you say "I am learning English" you don't have to right in that moment be actively learning English. Unlike the verb "ride," you can continue to learn forever.