I'll admit that I'm a bit new to PICO-8 and don't have a good handle on how it's built. Maybe other language support isn't trivial.

That said, I'd love to be able to use a subset of something like Ruby, Javascript, or Python in addition to the current Lua based subset.

Is there a chance we'll see other languages in the future?

This doesn't seem too likely to me.
But there's one thing in particular that might work right now. MoonScript is a language that compiles to Lua. With the token counting and the differences in base libraries it might not work perfectly, but hey, it's worth a shot.

I'm just making assumptions, but:

PICO-8 is made with LibSDL, which code is C/C++. And C/C++ can set up Lua bindings...
Maybe I'm wrong but I guess PICO-8 behaves that way: binding the Lua API we use on PICO-8 to execute native code. And, if I'm right, PICO-8 source code should be modified to support other languages, making it's implementation not trivial at all.

Transpiling (converting from a given language to Lua) is plausible. Not sure if it's desirable in all cases, but Haxe has taught me there is value in it. See YellowAferlife's Pico-8 target for Haxe: https://github.com/YellowAfterlife/hxpico8