I agree that e.g. locating .mo files is out of scope for a stdlib, but runtime format strings seems like something the stdlib could do. Because it does not, std::fmt is borderline useless for software which wants to support multiple languages.
Format strings, for example- are essentially compile time macros in rust in order to avoid all the fun kinds of string handling bugs that C’s implementation allowed- but that means no runtime customization without also defining were alternate formats live (and how they’re verified, etc). Not supporting multiple languages is a feature, not a bug. If you need multi language support, you need to a library to define those semantics in a way that fits or use case.