Can we do something against the locale problems that keep showing up
when updating?

Right now, after a half-finished update from guix master (half-finished because
once any package build breaks, the entire update will stop), I cannot use any
Umlauts because "xterm -u8" stops working as it should:
$ xterm -u8
Warning: locale not supported by C library, locale unchanged
$

Why does this keep happening?  Shouldn't the functional nature and modularity
of Guix keep using the old locales where those make sense and the new locales
where those make sense?

Environment:

$ locale -a
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_COLLATE to default locale: No such file or directory
C
POSIX
$ echo $LC_ALL
de_DE.utf8
$ echo $LANG
en_US.utf8
$ echo $GUIX_LOCPATH
/run/current-system/locale
$ ls $GUIX_LOCPATH/2.28/en_US.utf8/
LC_ADDRESS  LC_IDENTIFICATION  LC_MONETARY  LC_PAPER
LC_COLLATE  LC_MEASUREMENT     LC_NAME      LC_TELEPHONE
LC_CTYPE    LC_MESSAGES        LC_NUMERIC   LC_TIME
$ ls xxx
ls: Zugriff auf 'xxx' nicht möglich: Datei oder Verzeichnis nicht gefunden
$
^^ Note: Message is in German, as it should.  I left in the mangled Umlauts
for documentation purposes.