In particular, something where you can paste in a conlang specification (ideally one created since the model was trained), a sample text, and it will just spit out a basically correct translation into the conlang. I'm not counting something that requires lots of effort (with prompt and follow-up responses) to get working.
It must work on multiple conlangs with "interesting" grammars (i.e. it wouldn't count if it only worked on relexes of existing languages). The model must be one I am able to get access to or otherwise test.
Gemini 1.5 is supposedly able to learn to translate from English to Kalamang (an obscure natural language deliberately excluded from training) given just a grammar manual for it. I think the main question now is whether I'll be able to access (I'm planning to pay for access to the best version I can afford).