Stuff like input.value = "00000.1" doesn't preserve the leading zeros in an <input type=number lang=de>, for example.

In other browsers, it does, see bug 1586870 for an example of the sort of thing where this could matter.

I can't reproduce the issue you've mentioned here with:

<input type="number" lang="de">
<script>
  let input = document.querySelector("input");
  input.value = "00000.1";
  console.log(input.value); // 00000.1
</script>

I know there are issues around <input type="number">, but it's unclear to me what exact kind of problem should be addressed in this ticket and if that kind of problem is indeed lang related.

Yes, that won't be the case after bug 1622221 (and didn't use to be the case before 74).