multi-language by mail issues

multi-language works well on the whole site. But not by mail.
its a crazy and funny situation. i will tell you the situation.
A user form dubai sign up on my site in Englisch, well - works correct.
she get the welcome mail in Englisch, works correct.
she makes a request a friend to a user in Austria (me).
i am in Austria and the site load in German. works well.
but i get the mail-notification in englisch. and if i click on the link in the mail then loads the page in englisch.
thats for me no problem because i understand englisch.

now the funny way.
my father (german language) makes a request to the user form Dubai.
She see that and ask me whats that for mail. She understand only English.
But the notification by mail was in german. if she click on the link, then will load the webpage in German and not in Englisch.

Thats bad for peoples they not understand other languages.
The problem is the receiver user get the language form the send request user.