Languages not work - after install forum

Yesterday I installed the module forum. Change automatic the language of the entire page from Polish into English.
You can now set back the Polish language, where is the problem?
Recompile working properly but still is english site

use: 7.3.2 version

* I can't now set back the Polish language

Update problem:

When clearing the browser cache,the language is polish but after logging into the admin panel change to english

What happend? Cashe folder is empty cashe public also

Where to find the problem? Speaking the truth i did not do anything

The language is remembered for each user if they switch language while logged in. 

So it's possible that site language is changed when user log in.