I am building a wordpress site. On the upper-right hand corner of every page on the site I am going to have two (or more) flags, and clicking on any given flag should cause the site to display the text in the language corresponding to the given flag. In order to do this, I should be able to enter the text for WordPress pages, posts, blogrolls, etc... in all supported languages at once.
How does WordPress achieve this? (I also want to make one language the default so that if one text is missing in one language then it is displayed in the default language, but I would like the admin interface to be flexible enough so that any given text must be entered in both (or all supported) languages at once, with a single submit.
Do I need a plugin to achieve this behavior? How can I achieve such a setup?