I found an workaround as follows:
I have created 2 Guestusers: GuestDE (LOCALE: GERMAN) and GuestEN (LOCALE: ENGLISH) with a pswword and I have added the following link depending on the language to a static HTML page:
/logonOp.aw?domain=XXXX&userName=GuestEN&password=xxxx&testingMode=false
This loges the GuestEN (Group= Guests) autom. into the system and allows to change the language.