When using a frontend form to create a new user it is actually straight forward, because you can easily send a preferredLanguage attribute with the request that sets the language on creation (see this line in the user controller). However, if you want to use the CP form to create a user, there doesn't seem to be an out of the box solution for your problem. ...
Okay I got a response from the Solspace team, mentioning that they dont have this feature.
I have made a workaround and build a module for Craft, listening to the "After a submission is submitted" event.
This is what I did:
function (SubmitEvent $...