This is very strange. Janusz doesn't have the option to comment out the gettext native, so therefore he is not able to test the problems. I can simulate the Windows OS version with the gettext NOT installed to test the session left over.
After running the Dutch translation a while I have no left over session files. A couple was set to zero size though.
If you look in your error.log file (probably in the tmp folder in FA root, did you see a message:
[25-Jan-2012 13:12:14] 0::logout.php:47: session_destroy() [<a href='function.session-destroy'>function.session-destroy</a>]: Session object destruction failed
Of course with another date/time stamp. It should appear at the end. If you get such an error, there is a reason for the left over and we can start examine this error.
Also please tell us your version of PHP. There are some session_destroy() bugs in version 5.1.2.
/Joe