My only try with musl was a few years ago, I tried to install the musl version of Void out of curiosity, but the installer failed to set any locale, while this worked fine with the glibc version, so I didn't try any further.
https://docs.voidlinux.org/installation/live-images/guide.html#locale still says
Select your default locale settings. This option is for glibc only, as musl does not currently support locales.
It's probably possible to get some kind of locale support afterwards following the guide you mentioned.
However https://wiki.musl-libc.org/open-issues.html also still states that
Locale support is very limited, and barely works. Translation of LC_TIME is not possible because the key strings for ABMON_5 and MON_5 (“May”) are identical. Custom collation orders (LC_COLLATE) are not implemented at all, despite there always having been an intent to support them. LC_NUMERIC and LC_MONETARY also admit no variation by locale. Solving these problems requires a major overhaul, but the main missing prerequisite is involvement from users who want the functionality.
Building officially recognized locales for musl is also a recognized open problem. The bulk of the data should be derived mechanically from the Unicode CLDR where possible, but the CLDR seems to lack certain time format variants corresponding to the ones C/POSIX needs for nl_langinfo/strftime. This probably requires a great deal of manual work to remedy, ideally getting the missing formats added upstream with Unicode.
This honestly discourages me from trying, especially since I don't personally see a concrete benefit I would get from running musl instead of glibc.
To be honest I don't care about CLI language, I'm fine with that being in English, but I definitely want my DE and GUI programs to be able to use different languages, as well as proper numbers/time formatting.
What about you, does it work for you or do you just use everything in English?