If you follow an user who has a different language preference set on their profile, you will receive all related notifications about the user's activity in that language, regardless of your own setting.

Steps to reproduce

1. Create two users: userA and userB
2. Set the language for userA to German
3. Follow userA with userB
4. Create content/add comment with userA
5. Notice the notification is sent out in German, however the user setting for userA is English