Unicode is not included because currently they target ANSI C. Pure ANSI C.

Not POSIX.

Plain old C.

Until C11, unicode didn't exist in C. And it will be a long time before C11 is the default. Hell, there's lots of places where C99 is the exotic interloper.

What do you want to do that you cannot do because of the lack of Unicode support?

I'd imagine internationalized gamed would be a big one, although I'm not a games programmer or a lua user myself.

I had hoped that someone would mention what specific thing was missing, because as far as I know it's trivial to do anything one would want to do when making games already. That is, it is very easy to accept UTF-8 input, output UTF-8, munge UTF-8 strings together prior to outputting them, compare UTF-8 strings for equality, and so on. If the game needs UTF-8-aware strlen (for limiting the length of a field or alphabetizing things), it is like 10 lines of code.

World of Warcraft seems to struggle on (its UI is built of Lua)