Thanks to you too for your answers.
I myself am very close to having everything working. But I still encounter an issue :
[Info - 6:20:19 PM] Elixir sources not found (checking in /home/build/elixir). Code navigation to Elixir modules disabled.
I think I did everything correctly with asdf, following every step of the pugin instructions for Erlang and Elixir. But maybe I’m missing something… I saw that ElixirLS was designed to work with Elixir installed via asdf, so I wonder why it is trying to find Elixir in /home/build/elixir.
I saw here that someone cloned the Elixir repo on the previously mentioned path. But it seems a bit strange to have Elixir on my computer twice.