But if you wanna change the whole content of your pages into a second language, you must create a second language in admin and also create new pages in this second language.
For visitors to change language on your homepage you could use 'Top menu-first'. Create new page in both languages. In the 'Page form' use the 'Address' field and copy-paste the URL of the language. The homepage of every language has his own URL.
For example look at the URL of the OS homepage and change language from EN to PL.
Thanks guys (and/or girls). Yes I knew about the language_management. What i did not know was how the people could change from the english page, to another language page, same like in the admin page. I am now trying the source code of de admin page, and use the on-change method. What I also will try is the example of Guido.