[Witnesses Considered at the Marriage Contract]

Answer—and Allah grants success: It is necessary to rule the marriage contracts of such people valid, for several reasons:
1. Necessity, which has rulings that differ from those of choice. Thus Allah permitted the testimony of dhimmīs in the verse on bequests in Sūrat al-Māʾidah.
2. Ignorance. The scholars of the madhhab said: the madhhab of the ignorant person is that of the scholar he follows, so long as he does not violate consensus. Based on this, what is mentioned in the question does not violate consensus; indeed it accords with those who validate marriage without witnesses.
3. What appears to me is that the uprightness required of witnesses differs with madhhabs, religions, and societies. What counts as “upright” among the Zaydīs differs from the Ashʿarīs, and vice versa; what is upright among Jews differs from Christians, and vice versa; what is upright for one people or state differs from another, etc.
Accordingly, due to this variance, the contracts of marriage of those mentioned in the question are judged valid if the witnesses are considered upright in that society—even if they would not be considered upright in another society or by the adherents of another madhhab.
The proof for this third matter: When the Messenger of Allah (May Allah bless him and his family and grant them peace) came to Madīnah, he carried out the ḥadd of fornication upon two Jews, on the basis of testimony given by Jewish witnesses to their fornication.
As for the second part of the question—how should a person act if the people of that land appoint him to judge … etc.?—the answer follows from the third point above. It should be known here that it is not permissible to accept the testimony of members of certain sects against other sects, nor of members of one party against another party.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1