Hello
Have you checked the language settings of the "Email Receiver" in SU01. May be the language of user receiving email is set to Dutch. Also check the language settings of user under which "Extended Notification" background jobs are running.
Language administration can be done with transactions SMLT and reports RSCP0001, RSCP0004, RSCP0017, and RSCP0020. Reach out to Basis for help to check the settings if this is happening for all users.
If this issue is happening for 1-2 users then check language setting of these users.
Regards
Sandy