This might sound crazy but I think embodying the AI will be the long term solution here. When AI robots use language to relate their experiences and make predictions about the real world they are walking around in, it will prevent the model collapse problem. Their language might diverge from human language, but since we live in the same world translation should be possible.

Edit: Actually, I think that with a fairly small amount of auxilliary data, it could be ensured they keep the ability to speak English.