This plugin enables you to run your blog in a different language than the backend of your blog. So you can serve your blog using e.g. German as the de
Every language that is supported by WordPress can be chosen. You just need a copy of the translation files in your WordPress' language folder.
No you don't have to. I love qTranslate and use it for my own blog. qTranslate also offers the ability to switch the backend language. But if you don't want to publish the content of your blog in more than one language, you shouldn't use qTranslate. I created this plugin for users who only wan't to switch the backend language without the overhead of any multilingual plugin.