Translation:Does the waiter speak our language?
Not quite. I am not an English native, so I might get the English part wrong.
"Is the waiter speaking our language?" asks if that, what the waiter is right now speaking, is our language. Whereas
"Does the waiter speak our language?" asks if the waiter is able to speak our language.
I'm not an English native so ...
The sentence "Is the waiter speaking our language?" I would translate to "Ĉu la kelnero parolas nialingve?"
I would translate "nialingve" as "in our language," because it is a prepositional phrase describing how the person is speaking. In the given sentence, "nia lingvon" is a direct object, representing what the person is speaking. It's a subtle difference, but that's what makes the most grammatical sense to me.