> Wouldn't it work better to just write the thing in whatever language they can actually write in and then do a straightforward translation in a single pass?

Nontrivial translation tools are AI(neural net)-based tools (although not necessary LLM). Whole transformer neural net architecture was originally designed for translation.

I don't have a problem with people using these tools to translate their writing into languages they aren't fluent/literate in. It's a completely different dynamic vs. having them write for you.