I don't think we should make this change. Adding glibcLocales increases closure size for rxvt_unicode-with-plugins from 122M to 249M. This will need to be fixed in a more general (and lightweight) way, not by adding glibcLocales to every package. See ongoing discussion in #38991.

@xeji Thanks, I learned a lot today 😄 and I agree. My solution was way too agressive for my problem. On nixos, in fact the problem does not appear because LOCALE_ARCHIVE is exported correctly. But on other distributions, the variable is not set. Asking users to do it manually is not the best way to proceed, so it should be set by default. Would it make sense to add it to .nix-profile/etc/profile.d/nix.sh or is there a better place?

Update: My suggestion above is not good, it might interfere with applications on the host system in a bad way. Also, the glicLocales problem in this case is linked to perl, not to urxvt directiy I think.