I found this to be an interesting bug/feature, when people reply to a forum thread the email notification comes in using their language setting. Not a big deal, I suppose it's refreshing to get emails in german and spanish :D But I did want to point it out.
Are you sure?
The language will be selected for each user seperatly, based on the last choosen homepage language for each user.
So if you have changed your language to "German" on this homepage you will receive german notifications.