"I speak English."
Translation:Mówię po angielsku.
You should treat it as a whole phrase, 'po angielsku'. 'angielsku' is a form that doesn't exist anymore apart from this construction. This works kinda like 'English-style'. Apart from 'speaking languages', that phrase is also use in names of some dishes, like "ryba po grecku" (Greek-style fish) - a very popular dish in Poland, while Greeks apparently have no clue about that :D
What do you mean? without "mówię" ? rarely. that is short spoken answer to Po jakiemu mówisz? / W jakim języku mówisz?
No. Firstly, "mówi" is 3rd person singular, you need 1st person singular: "mówię".
Secondly and more importantly: I see that you tried to translate "I am speaking English" literally. That does not make any sense. You have to treat the Present Continuous construction... just as a construction. There isn't really any "to be" there, the only verb is "to speak".
"Jestem mówię" is like "I am I speak".