1) What is the formula I should prononce to stop a mut’a? Should I recite it in Arabic if the mut’a contract was recited in Arabic?

2) If the girl is far from me and we have agree it is over can I say it by voice message or do I have to call her in live? Can I either send her a written message or voice message?

1) You don’t need to recite in Arabic. Just inform her that rest of the period (duration) left I have gifted you

2) Yes if it is received by her then it is enough