Is it permissible to recite verses from the Qur’an in prayer and then repeat their meaning in English, whether aloud or in one’s heart, in order to understand them?

There is no harm in contemplating the meaning of a verse you recite in prayer, or in saying it to yourself, for this is part of reflecting and internalizing the meaning, provided you do not utter it aloud. This is based on the hadith of Mu'awiyah ibn al-Hakam al-Sulami, in which the Prophet, peace and blessings be upon him, said: "Indeed, this prayer is not suitable for any of the speech of people; rather, it is only for tasbeeh (glorifying Allah), takbeer (magnifying Allah), and recitation of the Qur'an." It is not permissible to recite the Qur'an in a language other than Arabic. It is permissible in supererogatory prayers to hold a mushaf (Qur'an) with an interpretive margin and look at it.