gnome-languages: set UTF-8 prefix, if the locale is UTF-8 but not explicit
This fixes an incompatibility between muslc and glibc, where muslc considers locales by default UTF-8, while glibc considers them ISO. This is a real problem when for example, running flatpaks under alpinelinux and postmarketOS.
Ref https://gitlab.alpinelinux.org/alpine/aports/-/issues/14851