Would "esperanta asocio" also work?
Yes, though one often likes to use the word "Esperanto" in these cases as it is about the language "Esperanto" and one would like for that word to be in the title of let's say an Esperanto association.
EN : Yes, but I thought there could exist only one noun in a noun groupe : "Mi parolas la esperantan lingvon" for exemple.
NO -- it is rejected by DUO. I tried.
NO. As of 2019 Jan 19, DUO rejects Esperanta Asocio. Just sayin'.