Steps to reproduce:

1. Inherit a bunch of users w/ language en_GB
2. Only install en_US
3. Upgrade
4. Try to edit a user to change their groups (mostly to disable a user by adding them to the disabled group and removing them from all other groups)
5. Click Update

Result:
See screenshot...

Expected:
If a field isn't being changed by the form submission, its value shouldn't be validated by the form and it shouldn't be deemed a change. This should allow users to make changes to Groups without editing a user to select a new language.