As of version 4.1.0, you can simply use the handy Language drop-down to change the language back and forth - no more URL editing necessary!
Setting the language of a page can be achieved by using the URL parameter showLanguage.
Add this parameter to the page URL in order to show the content in the chosen language.
For example: If you want to see the page's content in French, add this to your page URL: ?showLanguage=fr_FR
If you are not using a pretty URL with more than one parameter in it, you will have to add this to your page URL: &showLanguage=fr_FR
Please notice: This only works if the page already contains content in the selected language.