This fixes a regression introduced in
9954db4b59, where the realm's default
language would be ignored for users created via API/LDAP/SAML,
resulting in all such users having English as their default language.
The API/LDAP/SAML account creation code paths don't have a request,
and thus cannot pull default language from the user's browser.
We have the `realm.default_language` field intended for this use case,
but it was not being passed through the system.
Rather than pass `realm.default_language` through from each caller, we
make the low-level user creation code set this field, as that seems
more robust to the creation of future callers.
c6fe799369