If you have doubts about your ability in English and you say to me; "My English has been always like this?" It can be an implied question with the raising of the voice at the end of the sentence and so you are asking me to confirm your statement. "My English has been always like this, hasn't it?" clears up any ambiguity.

If I were talking about a student's ability to speak English, I'd say, "Their level of English has not changed". "Their level of English has remained constant". Or "Their ability to speak English has not changed". "Their ability to speak English has remained the same." Hope this helps

My English has always been like this. is correct English the question mark makes no sense with the sentence, you are making a statement not asking a question.