be happy to help point you in the right direction for this. just to clarify, are you looking to update the language in the footer of your emails where the "Safe Unsubscribe" link is located? if so the language of this section is pulled from the language setting in your account.
If this is not the language setting you are looking to update please let us know!
Changing the whole system to one language or the other is not helpful to bilingual organizations. We need more options to edit the buttons, and the event registration headers without changing the whole back end of the system.