It should be possible to send friend requests to other players independent of the users language. A website should never display technical error messages to the users. (Even though it helped me find the bug in this case.)
The backend returns an error saying that the language variable is not defined. Making it impossible to send a friend request to an user with another language.
(I'm german, if somebody wants to test it out )
In this line
$Friend_LNG = new Language($language);
should be replaced with
$Friend_LNG = new Language($row['lang']);
Also it should be considered to disable the debug error messages
