Is it permissible for an Imam to supplicate in English, knowing Arabic, and is it permissible for a non-Arab to supplicate in English at all times?

It is better for the supplication (du'a) to be in Arabic, because the Sharia was revealed in it. However, it is permissible to make supplication in another language, such as English, if the listeners do not understand Arabic, or if the Imam is not proficient in Arabic and no one else is available. If these reasons are absent, then it is better to make supplication in Arabic. Non-Arabs should learn Arabic because it is the language of the Qur'an.