When you have access to a project which contains multiple languages, you will see a globe icon in the project menu. Click this to open a pop-up window and select the language for the project you want to work in.
It is also possible to change the language in the URL by changing the culture. For example, if you are working in an English project you will see /en/ in the URL.
https://login.digitalcx.com/#PROJECT/DEMO/en/tasks
If you wanted to switch to the French project, you can replace 'en' with 'fr' in the URL.