It seems lika that language access rights are only checked in the dropdown menus in edit mode. If you access edit mode with a direct link to a page and a language you can access a language branch that you don't actually have access to, e.g. /cms/edit/default.aspx?id=12345&epslanguage=en&selectededitpaneltab=1. You can even edit the page and publish it.
Is this by design or a bug? For me it seems like a quite serious security bug.