People tend to contribute to the projects they already have the skills for.

People also tend to pick up new skills when they have a driving incentive to do so, like supporting a project they have a vested interest in seeing improved.

You need to learn the language's structures

Most of the bread and butter ones have analogues in other languages you should readily understand. More language-unique structures are rare; the more niche they are, the lower the odds your ability to contribute in a meaningful way hinges on your understanding of them.

you need to learn how the compiler works

You really don't, though? Modern compilers, particularly the Rust compiler, are designed to abstract away as much of the details of compilation as possible. If the project really does need to tickle the compiler a certain way to get it to build, it will almost certainly have a buildscript and/or a readme.

you need to learn the libraries that the FOSS project is using

This is true regardless of the language in use. I'm not sure why you brought it up.

you need to learn the security pitfalls for the language

I would imagine most of these language-specific security footguns are either A) so specific that you will never hit the conditions where they apply, B) are so blazingly obvious that code review will illuminate what you did wrong and you can learn how to fix it, or C) so obscure that even the project owner doesn't understand them, so you'd be at minimum matching the rest of the codebase quality.

Mind, I am not insinuating that one can simply bang out a whole new submodule of a project in an unfamiliar language with minimal learning time. Large contributions to large projects can be hard to make even when you're a veteran of the language in use, as the complexity of the project in and of itself can be its own massive barrier. But not every contribution needs to be big. And for most contributions, I don't believe the language is the most significant barrier to entry. It's a barrier, sure. But not the biggest one.

I'd wager it's not having a significant impact on the volume of contributions to Lemmy in particular.