I'm having a lot of fun contemplating what they're going to do next. Have they already learned each over's alphabets? Because if they have, then they could at least take notes on what the other person is saying and ask about it later, since it seems hard for them to even repeat what the other person is saying. For things that can't be demonstrated with a drawing, charades would be helpful, and I'd prioritize establishing words like, "again" or "slower" through charades. I have no idea how they established the word for "not", because in order to indicate that a thing is not something, you'd have to establish the something that the thing is not.
if they managed to estabilish Ryu and Myaa are their names, some basic form of negation is probably one of the first things that they'd do, because it's so useful.
MC can just point at himself, say Ryu, then point at Myaa and things around them, say not Ryu. Point at Myaa, say Myaa, point at himself and things around, say not Myaa. Should be pretty obvious to anyone that is familiar with the concept of negation.
Same method could be used later on for example to differentiate fruit and meat from the whole "food" category,
And you absolutely need it because you can use it to explain other more abstract things that you can't directly point at (dark = not light, night = not light sky/not sun sky, poisonous/spoiled = not food)