>
> Error: unsupported locales detected:
> eo.UTF-8/UTF-8 \
> You should choose from the list above the error.
>

Well, "eo.UTF-8/UTF-8" is indeed not a supported locale.  Why did you 
include it in your configuration?  "eo.UTF-8" (without a "/UTF-8") is 
definitely supported by glibc.  What do you get if you configure it like 
this:

i18n = { defaultLocale = "eo.UTF-8"; };

?

>
> FWIW, my system doesn’t have the obsolete X11 locale.alias file, and I 
> don’t use X11.
>

Then I guess Emacs doesn't use it, which is why your suggested fix worked 
on NixOS but wouldn't work on a Debian-based system.

>
> So it seems to me transparently clear that the encoding for the `eo' 
> locale is UTF-8, and yet somehow emacs has its own, separate opinions, 
> which don’t seem to be based on fact.
>

The story is a bit more complex than that, as you may have seen in the 
previous messages in this thread.