Change your blog's language based on the user's preferred language.
This plugin just dynamically switches the internal language setting for WordPress. This makes WordPress attempt to load up localization files to display things in that language. However to do this requires that a language file be present and that the language file contain a translation for each displayed string.
Posts cannot be translated in this method and if you aren't using the default WordPress theme, then you'll also need a translation file for it.
Change the preferred language setting in your browser to one that you have a language file. For example install the French translation file
fr_FR.mo and then change your browser's setting to French being the preferred language.
That's a job for another plugin. See Multilingual WordPress on the Codex for a list.
Try again in a minute. This plugin uses a little bit of caching to avoid looking at the languages directory on every page load. Results are cached for 60 seconds.