The thing is that for anything more complex than simple CRUD either side (frontend and backend) is such an inherently different problem domain that I don't see

The thing is that for anything more complex than simple CRUD either side (frontend and backend) is such an inherently different problem domain that I don't see that the programming language is the relevant factor. Because frontend and backend need a very different skillsets and both need a lot of knowledge and training to become good, there are very few true fullstack engineers. And I would bet that most if not all of those can handle work in multiple different programming languages.