ChangeLanguage is an extension for the Contao CMS which allows visitors to switch between different languages of your website.
Contao itself provides multilingual support by default, you can create unlimited number of page trees in different languages on either the same or different domains. ChangeLanguage simply adds the functionality to link pages together and present the website visitor with a list of available languages.
If your workflow consists of creating a page in one language and then duplicating it for translation, take a look at the i18n-duplication extension.