I see this in docs for CAS 6.5.x and newer but not in 6.3 or 6.4. I assume since it does not work in 6.3 it was introduced in 6.5. I know not everything is documented so not sure if this was available before 6.5
locale definition per service
{
"@class" : "org.apereo.cas.services.RegexRegisteredService",
"name" : "MyTheme",
"locale" : "de",
"id" : 1
}