I think this is a fair question. In my opinion, if you're developing a larger application then nearly all of your code will either be in the "racket" language. If you're keen on types, substantial fractions of this may be in the "typed/racket" language. It is true that all of the languages share a common substrate, and they can more or less all interoperate, but it's definitely not the case that a programmer would be likely to break their program into five equal pieces and use a different language for each one. You'll probably write your docs with scribble. Using different languages allows you to have a fundamentally different programming model, and while having that as a backdrop made things like the development of Typed Racket feasible, it isn't something that should prevent you from starting.

TL/DR: Just use Racket. If you find that you really wish that you had some other feature or style of programming, you can investigate whether there's a language that already provides that.

All of this is my opinion, of course.