The only language I know that is doing this is Falcon (
http://falconpl.org/ ). If there are stats for how many people use
this feature it would be interesting.
Plan 9 has had native Unicode support pretty much since day one (circa
1989). E.g.:
int Δt = time(0) - αρχη;
is valid C. There have been no issues with people using visually
similar characters in a way that causes confusion. Mind you, Plan 9
users tend to be militant minimalists, so nobody uses this feature
just because they can.
Sun's Fortress language also fully integrates Unicode into its
grammar.