Description of problem:
Consider adding support for changing system locale with virt-customize(1).
On systemd based systems this would be otherwise pretty straightforward (just adjust /etc/locale.conf) but if the system has only e.g. glibc-langpack-fi/langpacks-fi installed instead of glibc-langpack-en/langpacks-end or glibc-all-langpacks, then switching from fi_FI.UTF-8 to en_US.UTF-8 properly might not be that trivial.
More exotic cases like using "%packages --instLangs=en_US" in kickstart or configurations like below may also make this a bit more involved. One option might of course just to document these limitations.
echo "%_install_langs en_US" > /etc/rpm/macros.image-language-conf
echo "override_install_langs=en_US" >> /etc/yum.conf
We can install new packages, or as you say document in the manual
that changing the locale might need to be accompanied by using
‘--install’ + a suitable langpack. It would be nicer to do the
right thing automatically.