First I would get the accept_language, either like the first poster mentioned, or simply using:

$ENV{HTTP_ACCEPT_LANGUAGE}
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The second step, is I would override your path for your templates. Both HTML::Template and Template::Toolkit allow you to specify an arrayref of paths to look for your templates. I would unshift your language override directory on to the front of your path arrayref (in HTML::Template - INCLUDE_PATH in TT). That way, it will find you language override templates - but will fall back to your default templates if a language override can't be found.

my $base_dir_abs = "/var/www/templates";
my $default_lang = 'en';
my $lang         = lc $ENV{'HTTP_ACCEPT_LANGUAGE'}
my @dirs = ("$base_dir_abs/$default_lang");

if ($lang
    && $lang ne $default_lang
    && $lang =~ /^(\w{2})/ # may look like "en-us"
    && -d "$base_dir_abs/$1") {
    unshift @dirs, "$base_dir_abs/$1";
}

my $tt = Template->new({
    INCLUDE_PATH => \@dirs
});
$tt->process($file);

OR

my $ht = HTML::Template->new(
    filename => $file,
    path     => \@dirs,
);
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Update: adding specification for how to store your content.

# Your directory structure may look like
/var/www/templates/en/my_cgi/index.html
/var/www/templates/en/my_cgi/step2.html
/var/www/templates/en/my_cgi/step3.html
/var/www/templates/en/another_cgi/index.html

/var/www/templates/sp/my_cgi/index.html
/var/www/templates/sp/my_cgi/step2.html
/var/www/templates/sp/another_cgi/index.html
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If the CGI is passed HTTP_ACCEPT_LANGUAGE of sp and $file = 'my_cgi/index.html'; then you'll get /var/www/templates/sp/my_cgi/index.html. If you call it with a language other than "sp" then you'll always get /var/www/templates/en/my_cgi/index.html (well - always until you override my_cgi/index.html in that other language).

Likewise for templates such as my_cgi/step3.html that don't exist in sp - you'll always fall back and get en/my_cgi/step3.html.