Make language code extraction more robust.

There are three letter language codes, e.g. fur.

config/chroot_local-includes/usr/local/lib/tails-shell-library/localization.sh

 #!/bin/sh
 
+language_code_from_locale () {
+    echo "${1}" | sed "s,\(_\|\.\).*$,,"
+}
+
 localize_tails_doc_page () {
     local page="${1}"
-    local lang_code="$(echo ${LANG} | head -c 2)"
+    local lang_code="$(language_code_from_locale "${LANG}")"
     local try_page
     for locale in "${lang_code}" "en"; do
         try_page="${page}.${locale}.html"