I gave it a try out of curiosity with Rust, which you might reasonably expect to be well supported. But it didn't start a language server. So I followed the docs to find out how and was none the wiser. A tad undercooked, I'd say.
Not sure if this is still necessary, but I needed this workaround in ~/.config/helix/languages.toml:
language-server = { command = "rustup", args = ["run", "nightly", "rust-analyzer"]}
From a brief experiment basic lsp usage seems to work in helix without any config changes, so perhaps that's changed since you set it up.
It also only works on Rust projects, not single files. You have to open helix with the path to the project folder for it to work.
Documentation is missing, it's a bit scattered