Making it map to every single language is always going to be hard, but most (mainstream) languages come with support for these kind of things in the language itself or standard library. That's not a good reason to add more though. It also pushes the complexity the complexity to the people using TOML in their programs, rather than the people writing TOML parsers: in some languages like Python or JavaScript you can "just" use a string and it's nullable, but in other languages you have to muck about with pointers, special types, options, etc.
