Changing default language.

[vxda]

Hi, some time ago i developed hejtuje.com, its default language is Polish, as i was building site i was adding translation for second language English.
Now i choose not to use multisite module and i just copied website to new folder for hate.it. Its default language is still Polish with translation of english there. I would like to make english my default for this site. is it possible ? I tryed changing language for guest user but it didnt work.
If its important i updated PW from 2.4 to 2.5.

Any ideas how to change default to English  ?

Thanks.