What should one who does not understand the language of the Friday sermon do during the sermon? Should he read the Qur'an and make dhikr, or sit and listen to something he does not understand?

If you listen to the sermon and do not understand it, you are still on the right path (or doing good). Some scholars have given a concession to those who do not understand the language of the sermon to occupy themselves with dhikr (remembrance of God) or reading, provided they do not disturb those who are listening. However, the best approach is to listen, even if you do not understand.