Here, the "了" make the answer softly. Like this dialogue： -你还有钱吗？ -没有了 we put ”了“ behind negative answer to make the tone softly. The meaning of the sentence would not change if we remove "了".

if you use 了 at the end of this sentence, then the answer is kind of like "I cannot speak other languages yet" and the modal particle 了 makes the answer more euphemistic and humble. Without a 了 in the end, the sentence means"No, I can't".

Actually it is the same meaning for having the "了“ or not. it is like a modal particle to show his honesty and humbleness. However, if you really want to make a difference, i can try to explain it. since you ask "除了”, that means you are asking for the extra kinds and amount of languages, the reply with 了 actually is putting an end to the possibility of him to grasp extra amount of languages that you thought he may have. Hope that it can help you~!^_^

No meaning, just like "啊，哦，额". Chiness is a amazing language. Hope having fun

it has not sense in this sentence.