Again, given the limits of LLMs (stochastic, rapidly changing, everything's a hallucination, widely known prose issues) I am skeptical that you really care that much about optimal prose. I could believe it's one of the things that you care about, but at a pretty low priority level.
Taking you at your word, though, I'd be interested to see what you think of the watermarking technology in a blind A/B test.
What's your evidence that it will result in worse translations?
I'm skeptical that such a thing as a universally optimal translation exists in cases beyond the trivial. But if it does, I see no reason to think LLMs are anywhere close to it, so I think nobody will be able to tell the difference with watermarking.
That's certainly true for code. LLM code is at best mediocre. There is oceans of room to subtly watermark generated code without practical impact.
Taking you at your word, though, I'd be interested to see what you think of the watermarking technology in a blind A/B test.