How about changing the lang.php file in this directory, /home/httpd/html/horde/config. I just tested this with IE 6 and here is what I am seeing.
No matter what default language I set, I always see it in english. But, here is why, in IE under tools, general, languages, my default is set to en_US. If I remove this, then go back to webmail, then the default language that I set in the lang.php file shows up as expected. Once I re-add my defaults back to IE, then english is displayed.
Here is an snippet from the lang.php file
<snip>
/* The langauge to fall back on if we cannot determine one any other
way (user choice, preferences, HTTP_ACCEPT_LANGUAGE). */
$nls['defaults']['language'] = 'en_US';
</snip>
Good Luck,
JB