I am using Group Policy Registry settings to setup en-US welcome screen input language by default on domain computers. There are only two registry values in single key:
I am sure that default input language is EN on welcome screen. It is very usable because usernames are also English.
But this policy doesn't work on Win10 by default, "from box". And that's why.
There is undocumented feature in Windows 8/8.1/10. It performs automatically copying user language settings to login screen.
This feature can be disabled by Local or Domain GPO here:
Computer configuration/Administrative Templates/System/Locale Services/
Disallow copying of user input methods to the system account for sign-in
"This policy prevents automatic copying of user input methods to the system account for use on the sign-in screen. The user is restricted to the set of input methods that are enabled in the system account. Note this does not affect the availability of user input methods on the lock screen or with the UAC prompt. If the policy is Enabled, then the user will get input methods enabled for the system account on the sign-in page. If the policy is Disabled or Not Configured, then the user will be able to use input methods enabled for their user account on the sign-in page."
Just enable it, and you can control input language on welcome screen by only two registry values.