Problem with mod_negotiation

[Petr Hracek <phracek2@xxxxxxxxx>]

Dear users,

In my browser IE 6.0 I have set language priority as:
de-at
en-us

Apache version which is used on our server is: apache2-2.2.3

Apache configuration stored in /etc/apache2/conf/foo.conf has row:
include /opt/apache/conf/mls.conf

which contains following rows:
AddLanguage en .en_US
AddLanguage en-us .en_US
AddLanguage en-gb .en_GB
AddLanguage de-at .de
AddLanguage de-ch .de
AddLanguage de-de .de
AddLanguage de-DE .de
AddLanguage de .de

LanguagePriority en-us en en-gb de de-de de-at

Unfortunatelly when I go to the the index page http://<IP_Address>/
then page is shown in english.
When instead of de-at is de then all works fine.

All html files stored on the server have extensions like:
html.en_US or html.de

SSI which are used in this html files are defined as
<!-- #include virtual="public.html"-->
There are some <A href> flag where is defind URL path like /public/test.html>

Could you please tell me where I have made a mistake?

Thank you in advance
-- 
Best Regards / S pozdravem
Petr Hracek