Hi
I have two languages on my site polis and english.
I would like to setup URLs

domain.com/ for polish language
domain.com/en/ for english

That mean i want to add language suffix to english language only .
Is it possible??

Or another way

domain.com for polish language
en.domain.com for english

how to do it?

I use joomsef 2.3

This will be possible with the next release of JoomSEF, but however, only with JoomSEF 3.x.
We do not plan to add this functionality to 2.x as the support for Joomla 1.0.x is to be ended soon.