In your RDS farm you’ll want to ensure that users settings are applied automatically, this is especially true for RemoteApp deployments where users don’t have any access to settings.
While this could be accomplished using startup scripts or GP Preferences my preferred method is a GPO that only allows the user to use a specific language.
The policies are created per language you want to apply and I usually try to link them to country specific usergroups if they exist.
The policy to configure is the following:
User Configuration \ Policies \ Administrative Templates \ Control Panel \ Regional and Language Options \ Restricts the UI languages Windows should use for the selected user
After the policy has been applied and replicated users logging into the environment should have the policy applied.