By reviewing your concern, I understand that you want to
change your display language for the service account using the Ad Manager API which is not possible/available. I will raise a request for this feature, however, kindly note that I am unable to comment on any timelines for its exposure in the API. With that being said, I would recommend you to follow our
Ads Developer Blog and
release notes for announcements about new Ad Manager API releases and features.
Please let us know if you still need any further assistance.