Is it permissible to repeat the dhikr (remembrance/invocations) recited in Arabic during prayer, in English in one's mind afterward, in order to increase khushu' (devotional humility) and concentration?

There is no harm in recalling the meanings of the words and remembrances of prayer in English after uttering them in Arabic. This is a form of contemplation and grasping the meaning, provided that you do not vocalize them, but rather keep them to yourself. This is because a person performing prayer is prohibited from uttering these words in a language other than Arabic if they are capable of pronouncing them.