Languages are relatively “small” in memory, even a C++ compiler and IDE will fit on a USB (4GB if you’re using CLion, but a smaller IDE or just the compiler is easily <1GB).
In terms of features:
For a production language, I’d say Elm. I don’t like Elm, but I think it’s by far the simplest language which has any sort of industry adoption (albeit not much). I like in Elm the idea of a Haskell-like language without the insane complexity of Haskell, opinionated tooling, and great debugability.
Second-place is Lua. It’s far more popular and still very simple. It’s also not opinionated and thus doesn’t have deal-breaker flaws. I think almost anyone else would rate Lua over Elm, the only reason I put Elm first is because of what it tried to be (and because it has static types).
If you don’t care about real-world use, Scheme. Scheme is probably the “correct” answer to this question, it’s by far the simplest (particularly if you ignore R6RS and just implement the bare minimum to make it Turing-complete). Second-place is ML, especially if you skip some of the “unnecessary” features even SML has, like exceptions (idk if there are more unnecessary examples for SML like OCaml’s objects).
Personally I just think Scheme and ML are a bit too simple and if you try to write anything more than a tiny program, you’ll have to come up with your own design guidelines and library functions which is hard. They also don’t have much resources and tooling. Which is why I put the languages with real-world use first. Elm and Lua seem to stay simple even including their standard library and best practices.
Another option is earlier languages, like C99, Go before generics, Java and JavaScript before 2010, etc. These languages are good targets if you want to write an interpreter or compiler, since (particularly if writing a compiler) if your project gets useful you
can build up to the new versions later on, and (particularly if using them to write your own programs) you have all the production-grade support of the newer versions.
OCaml, C, and Go were contenders, but the latest versions aren’t actually simple, just not as many features as as C# 12, Java 20, ECMAScript 2023, C++ 2023, Rust, etc. Elm has been locked in time, and is permanently dedicated to providing the bare minimum essentials you’d need to build a maintainable web application (I think in practice it provides even less than the bare minimum…). Lua gets new features but very infrequently; forget incompatibility, you could have Lua code written 10 years ago which wouldn’t be written differently today (see the version history, https://www.lua.org/versions.html). Off the top of my head, I can’t think of any other languages like these which have any sort of mainstream adoption, except maybe some embedded or legacy ones I’ve forgotten or not aware of.