I'm thinking about differen string representations used by different libraries. Is this not true?

Honestly all I've encountered default strings (and people shy away from those now) and LibICU stuff. I think ICU has become a defacto standard but I have been out of C for a few years.

This tends to be important for strings you're going to show to the user (or input you gather from them), maybe most of the C in the world isn't aimed at interfaces like that...