I have slightly changed the sentence in the question title to be both short and clear. The actual sentence I'm asking about is:
It would be nice of you to comfort that person in all the languages you can speak___.
I'm not sure of the proposition that would fill the blank. I thought of "speak in" but didn't find such a phrasal verb when I have searched it on the internet.
I have also searched for the possible prepositions that can come after the verb "speak" and found (with, of, and to) which came with different phrasing of sentences.