Is it permissible to supplicate in the Witr prayer in congregation in a language other than Arabic—that is, in the language of the people—so that the supplication may be understood and the worshippers may benefit from it, given that the imam is proficient in Arabic but the majority of the worshippers do not understand it?

As long as the Imam is able to supplicate in Arabic, he should not supplicate in any other language; because the Qunut supplication is among the reported supplications in prayer, and some scholars have prohibited supplication in other than Arabic for those who are able. It is permissible to translate supplications and recommended remembrances like Qunut, the takbirat of transition, and the glorifications of bowing and prostration for one who is unable, but not for one who is able. In An-Nawawi's saying: The most correct view is the permissibility of translation for the one who is unable and its prohibition for the one who is able.