Well. For small projects or even larger open source projects I would expect the translators to just clone a ts file (which would basically just be JSON, but with comments, template strings, function calls), change the translations and contribute the file as-is.

For large projects it would still be possible to have a utility program export a translation to CSV/whatever and re-create a ts file from it when it comes back.

Any mistakes made by the translator would immediately show up when building the program or just visiting the file in a code editor.