Is it permissible to invoke blessings upon the Prophet in a language other than Arabic, while preserving the same meaning, and would the reward differ in this case?

Praying upon the Prophet is a form of supplication (du'a), and supplication is permissible in Arabic and other languages. The Malikis stipulated that it is disliked to supplicate in a language other than Arabic within the prayer, but outside of it, there is no dislike. Therefore, what you do by remembering the Prophet (peace and blessings be upon him) and praying upon him in another language is permissible and there is no harm in it, even if you are proficient in Arabic. As for the reward, no text has been narrated differentiating the reward for remembrance in Arabic or other languages.