ChatGPT already speaks Lojban, or at least enough to fool me.

It appears not: "vaguely grammatical and has some of the right words" according to someone who actually knows:

https://www.reddit.com/r/lojban/comments/12i0d0i/chatgpt_app...

Not surprising, given it would have seen many orders of magnitude less Lojban training data than its English input (basically two books and maybe a few megabytes of web pages).

The word by word translation sounds like it's trying to say that it isn't very competent at lojban, but that it can try to learn lojban if you provide it with parallel examples. All this said in broken lojban, as expected. Quite reasonable, actually.

I gave much larger snippets from the translation of Alice in Wonderland, and it "translated" them without complaints, but it was about half gibberish.

Same thing with modern takes on Old Norse.

Any language existing prior to 2021 isn't gonna be very useful for testing its improv abilities, since they're likely all in the training data.