The first part of the email subject is build using the user language of the one who @ mentioned instead of the recipient(s) language.

This happens because the following line of code in email_api.php is called one single time using the current language settings

$t_subject = sprintf( lang_get( 'mentioned_in' ), $t_subject );

Other parts of the email message are build in the loop for the recipients after
lang_push( user_pref_get_language( $t_mention_user_id, $t_project_id ) );

Subject has to be splitted in two parts for a fix with good performance.