If you start with some working software, you could make an LLM generate a lot of tests for the existing functionality and ensure they pass against the existing software and have excellent test coverage. Generating tests and specifications from existing software is relatively easy. It's very tedious to do manually but LLMs excel at that type of job.

Once you have that, you port over the tests to a new language and generate an implementation that passes all those tests. You might want to do some reviews of the tests but it's a good approach. It will likely result in bug for bug compatible software.

Where it gets interesting is figuring out what to do with all the bugs you might find along the way.