Hi,
The error message means that you tried to install a JoomSEF extension but your JoomSEF was not installed properly. You should be able to fix the problem by copying the file /administrator/components/com_sef/adapters/sef_ext.php to /libraries/joomla/installer/adapters/sef_ext.php.
For K2 you need to use the K2 extension for JoomSEF purchased at:
www.artio.net/e-shop/joomsef/joomsef-ext...-joomsef-4-extension
Then you can configure the SEF URL structure to suit your needs.
For Joomla articles JoomSEF can generate the URLs you require out of the box, you just need to configure it in JoomSEF's Content extension.
JoomSEF supports multilanguage websites. Yes, you need to disable the System - Language Filter plugin and enable JoomSEF's language management system.
I'm sorry but I don't understand the "multi - languages page switch" part of the question. JoomSEF supports Joomla's Menu Associations feature, so if you associate corresponding menu items in different language, the URLs will link to correct menu items in different language.