Wordpress multilanguage plugin [closed]
Gengo looked interesting, but development seems to have stopped.
But the qtranslate plugin (author's page, plugin page) offers the things you need. It not only manages article translations but also switches the complete site (“user interface”) language.
Articles (including titles) can be stored in several languages. If you need a translation elsewhere (i.e. a widget title), you can use special markup (language tags): <!--:en-->english<!--:--><!--:de-->Deutsch<!--:-->
I'm still testing it myself, but it mostly works alright. There may be surprised (e.g. date fields), so better test it on a offline copy of your site.
Also, check this older stackoverflow question
I have built several multilingual sites and I use the WPML plugin and have found it to be best option out there. http://wpml.org/