I translated mod and noticed that mail is sent using language of user who sends it, instead language of user who receives it.

I know that it may look a bit strange, especially as there aren't options to compose mail in different languages, so it beats purpose of having ability to send mail to users who speak different languages, but anyway, correct way of doing it would be to send mail with phrases from receivers point of view, not senders. Ok, maybe I'm nit picking, but now mods could function only with single language. And it should respect language of user who receives mail, not of user who sends it.

It also could be feature request, to enable people from really multilingual communities to use this mod? Having ability to create same mail in different languages used in forum and send them at the same time in same campaign but checking receiving user language and sending mail in it?

Anyway, in the manual you should mention that phrases in mail would be from users who sends mail and that this mod isn't for use in really multilingual communities. It's this page in manual that should have this information: Phrases

Unfortunately I'm not sure this is fixable.

I'll look, maybe vBulletin themselves has a way of doing it with their phrases, but I wouldn't hold my breath