Translating pages, only the last version remains I am setting up my first Drupal site, with Drupal 7, which is multilingual; I thought after surprisingly much work I had set it up correctly.
I can choose a language for each page, and translate pages, but when I translate a page, all translations of that page will have the same text: the last one I entered. So the text of the other language versions is overwritten by the last edited one. Did I do anything wrong? Is there a bug? 

You may have a look here.

The login credentials, if you want to test it are: test, test.
The problem is the 'synchronize translations' submodule of i18n was enabled.
I took the privilege of disabling it on your test site. 
I know multilanguage is a tricky part in D7 but you should start with the basics of multilanguage by enabling locale, content translation, Internationalization, block languages, multilingual content and string translation. Just to get a feeling of what you're doing.
Later on you can activate some other submodules of the i18n module but in my opinion it is never a good idea to activate all at once. Activating to many modules at once can result in losing control of your installation like you're having right now.