How can a young man convince his mother to approve his marriage to a girl of a different nationality, knowing her desire for him to choose a girl of the same nationality and her stubbornness?

It is not permissible for a woman to be in a romantic relationship with a man who is a stranger to her. This relationship must be severed, and she must repent. Striving for marriage is a good thing. The young man should persuade his mother through supplication and seek the help of those who have influence with her. If she does not respond, then obedience to the mother takes precedence, unless the young man fears temptation for himself, in which case he is permitted to marry without her consent, and then he should strive to please her. If the marriage does not take place, then one must turn one's heart away from it and submit to Allah's decree, for what is considered in compatibility for marriage is religiosity and character, not material and social differences.