how to change everything to english language ?
 

During my distro installation I chose Polish language (my native). Now I would like my linux to work like if I would choose english during installation. How do i do that ?

Very important for me is that ALL would be in english - not only newly installed software but everything from the beginning to the end.

Thank You for replying :)

Well you can set the language of user accounts in KDE or Gnome in the config menus, which will make most things in English ... but some apps have their own localisations, so if you set up say, Firefox with the Polish localisation you may not have the English localisation installed. Therefore how much will be in English depends on what you installed.

Don't get me wrong but from what You wrote I still don't know how to do all this.
Of course... I could install my distribution again but the point is I want to learn linux, not only reinstall it whenever I mess something up. If You have any information on how to do it, please post it here. I will read all of it carefully (I really want to learn linux) ;)
Step-by-step instructions or any kind of descriptions would be best for my level of linux understanding (almost zero) :study: