"Esperanto speakers are speaking Esperanto in a café."
Translation:Esperantistoj parolas Esperanton en kafejo.
So I take it that the "tist" in Esperantistoj means person or something like that, right?
esper'i is to hope
-ist- usually means someone working with this
-o denotes the word being a noun
' to mark the "word parts")
[hope, hoping] + [does this right now] + [this is "their job"] + [this word is a noun],
i.e. (my interpretation) "someone keeps the hope going"
i.e. an Esperantist.