Emails need to be manually translated, they are a one off translation, i.e. they are not user but Forum specific so you have to change the email templates themselves if you want to translate them. They live in the views\emails

ALL users will get emails in the language you translate them into not in their selected language, so you may want to use dual language email. This is because emails are processed in the background by a custom job processor and know nothing about users

No, the background processor does not interact with clients browser it is a totally separate process independent from the forum code, it just knows about the data required for the email.

I could add the users lang to the email object so the processor could deal with Multilanguage emails, but this is a biggish change as it would also require changes to the userprofile (lang is currently stored in cookie and would need adding to the members table in order to work (you can't read someone else's cookie if you send an email to them)) so not going to happen just yet.

May be able to do this when I release the userfields mod, will look into it.