Is it sufficient for a marriage to be Islamically legitimate for a young Muslim man living in Europe, if it is merely declared before Muslim witnesses in a mosque, without a guardian (wali) or a court?

A guardian is a prerequisite for the validity of marriage. If the girl's guardian is absent, he must be present or appoint a proxy. If she has no Muslim guardian, the Muslim judge or the official responsible for Muslim affairs, or a just Muslim man with her permission, should marry her off. However, if the girl is a woman of the Book, it is permissible to marry her on condition of chastity, and her guardian's proxy must be from her own religion, or her bishops may marry her off, or the Muslim ruler. It is preferable to marry a Muslim woman. If marriage is not feasible, it is advisable to fast and avoid anything that incites lust.