The issue is always the parts where the two language semantics differ too much. And then, you usually have too much legacy code that would be impacted by change

The issue is always the parts where the two language semantics differ too much. And then, you usually have too much legacy code that would be impacted by changes in those areas. That's the hard part. You lost when that code lies beyond some organizational boundary, e.g., a separate project that has is too big in its own right to refactor in one go, or where the maintainers have sufficient clout to resist such a change. Such a transformation would effectively entail a fork of that project.