In English, "I speak," is usually an incomplete thought. I speak when? I speak how? I speak where? I speak for what reason or to whom?
If the moment is now, then "I am speaking." That's the proper construction.
An example of when "I speak," is proper is if you're answering someone's question.
"Who speaks on Tuesday nights?"
No, that's a broken sentence like "I am I speak." You're confusing the English verb "am" (a conjugation of "to be") which means "in the state of being something" (am hungry, am blond, am healthy). This translates to jestem in Polish. You're confusing it with the English verb "am speaking" (a conjugation of "to speak"), which translates to mówię.