Where engleza and where engleză?
Eu vorbesc engleza and Eu vorbesc engleză are both fine translations for "I speak English".
What is the function of limba?
Limba means language, so when you say "limba româna" it means "Romanian language", while "româna" is just "romanian"
So why not "limba englaza"?
Because it is implied, and someone can understand that you are referring to the language. It is basically the same structure as in English - "I speak English" and "I speak the English language". Same meaning, less words for the former.