The following WordPress plugin “Codestyling Localisation” allows you to translate your plugins in your site.
This is not suitable for a multilingual site (multiple languages and one backend) It is only suitable for translating dedicated sites and specifically the plugins and tools
This means you can have the backend of wordpress in english WHILST you translate the frontend.
This plugin does not show you the final step which is to go to > settings and to the bottom of the general settings section where you can select the final language you wish the site to be translated into.
