In search.cgi after:

@values = &split_decode($line);

Try adding something like:

if ($in{'Language'} and ($in{'Language'} eq $values[$db_language])) {
push (@{$link_results{$values[$db_category]}}, @values);
$numhits++;
next LINE;
}

which basically says if $in{'Language'} is not null, and it matches $values[$db_language] (make sure you've defined $db_language, then add it as a match and move on to the next.

If you want AND matching (i.e. has to match language and search term) then try something like:

if ($in{'Language'}) {
($in{'Language'} eq $values[$db_language]) or next LINE;
}

Hope that helps,

Alex