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.
Make sure you have rust-analyzer installed, and check `hx --health` to see that Helix finds it. 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"]} This should not be necessary when rust-analyzer is stabilized (may be already), but last time I tried it still needed nightly.
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.