There are two, and mixing them up is easy. One changes the interface for you. The other changes the emails your signers get.
Your interface language
The EN dropdown at the top right of any page. It affects what you see and nothing else. Change it as often as you like: nobody else is affected.
Company language
Your name at the bottom of the sidebar → Settings → Language. This one is the default language for emails swipesign sends to your signers, and the default for everyone on your team.
Which one your signer actually gets
Your company language sets the default. It's the language of the invitation email, reminders, and the signing screens, so if you mostly send to German-speaking clients, set it to German and stop thinking about it.
Signers can switch the language themselves once they open the document, so getting the default wrong is inconvenient rather than fatal.