The locales (translations) for the SquirrelMail package are provided in
the package "squirrelmail-locales" on which the "squirrelmail" package
recommends. You need to install that package as well if you want to use
SquirrelMail in languages other than English.

You may need to reconfigure locales with:
  dpkg-reconfigure locales
to make alternative languages work.

In some cases it might also be needed to restart Apache after you've
upgraded or added locales, because of some internal caching of locale
information by Apache.

=== Locale with correct charset needs to be configured ===

The locale of the language file you want to use has to be configured
on the server.
E.g., to make the "French" translation work, you need to setup the
fr_FR.ISO-8859-1 locale. Currently it will not work if you only have
the fr_FR.UTF-8 locale enabled, you need the ISO-8859-1 one as well.

You can generate locales using:
  locale-gen LOCALE
You can see which locale names get used for a given language in the
setup.php file of the translations,
e.g. /usr/share/squirrelmail/locale/fr_FR/setup.php for fr_FR.
See the "locale" program to display installed locales, e.g. "locale -a -v".