In MDL-60736 a new parameter was added to WebServices to allow defining the language to use when returning contents. If this language isn't installed in the site, Moodle should try to use the parent language instead.

E.g. if I send "de_du" to a site and that site doesn't have "de_du" installed but it has the "de" language installed, then the contents should be returned using the "de" language. If there is no parent language or it isn't installed either then it should fallback to the current behaviour.