changing the language dynamicly

In ckfinder.html there is:

var CKFConfig =
{
   AutoDetectLanguage   : true,
   DefaultLanguage      : 'en',
   Languages         : 'en,de,es,es-mx,fr,it,lv,pl,pt-br,ru,sk,zh-cn,zh-tw',

   SkinPath         : CKF_CORE_PATH + 'skins/default/',
   DirectDownload      : false
};

you can set AutoDetectLanguage to false, rename ckfinder.html to ckfinder.php, replace "ckfinder.html" with "ckfinder.php" in ckfinder.js and/or in core/ckfinder_phpX.php and then use PHP code to set DefaultLanguage using session variable.

An alternative would be to set in .htaccess to execute PHP code also in HTML files, this way you could avoid renaming ckfinder.hmtl file (but this approach also has some drawbacks).