As far as I understand, Request Language is a (locked) custom field. I can see all custom fields for a request type with
GET /servicedeskapi/servicedesk/2/requesttype/{requestTypeId}/fieldbut Request Language isn't one of them.
I understand that since the request is submitted on behalf of a user (in this case the user to whom the api key belongs) and as such, the Request Language should be a field on the request form so it shows in the list of custom fields (correct?) but when I try to add it to the form I get the following warning

Is there a way to provide a preset value? And is this the correct approach?
Another option would probably be to change the default language of the user to whom the api key belongs (haven't tried it yet so not sure if this is correct) but since we forward requests from our app through our backend, it would be nice if we can inherit the user's app language when creating the ticket.
So what is the advised way to set the request language using the REST api?
Many thanks