P3876R2 is working on that (or rather, making the first steps toward that support by letting you use char8_t in std::to_chars). I doubt anyone cares enough about <iostream> to make that work with char8_t given that it's basically abandonedware. We don't add support to it for any of our new types (and we can't without breaking ABI), so it just keeps getting worse compared to <print> and <format>.

Use char and make sure your environment is UTF-8. If you use char8_t, you're doing it in spite of the lack of support, maybe in hopes of future-proofing for when the standard will have better Unicode support. I suspect things will improve drastically in C++29.