In order to change the Flipsnack language, please go to flipsnack.com and scroll down to the bottom of the page. In the right part of the footer you will find the language changer drop-down menu. The default language will always be English.
You can have Flipsnack displayed in the following languages: English, German, French, Spanish, Portuguese, Dutch and Italian.
If you want to change the language of the Fullscreen button, check out this article.