We have an EPiServer CMS 5 site in 9 languages with the following language codes:

English - en-us
French - fr
Italian - it
Spanish - es
Portuguse - pt
Polish - pl
German - de
Turkish - tr
Russian - ru

This works fine for IE6 users who have browsers set to the 2 letter language codes.

However, with the increased adoption of IE7, we now need to managed the following 4 letter codes:

fr-fr
it_it
es_es
pt_pt
pl_pl
de_de
tr_tr
ru_ru

At the moment, all these language settings are defaulting to en-us.

Does anyone know how we can handle this problem so that an fr_fr browser redirects to the fr content ?