The locales handling was completely broken in countrychooser...See #230463
Indeed this came from me confusing locale and the language list.
After reading a bit about LANG and LANGUAGE variables with pointers
recently given in -i18n, I decided and commited what follows:
-the debian-installer/locale value is now untouched by countrychooser
-provided xx is the language code and YY the country code, the
debian-installer/language value inherited from languagechooser is
prepended with xx_YY.
User chooses French (France) at languagechooser:
User chooses USA as country in countrychooser:
If the user chooses FR as country, then the languagelist is NOT
modified (not prepended again with fr_FR).
This is still far from perfect (the interest of the language list may
be minimal) and this does not solve the locale problem, but at least,
this won't break anything..:-)