Settings → Language.
Your screen
Yours alone. Everyone else on the team keeps their own — in one company the owner may want Spanish and the accountant English.
What your customers receive
Invoices, estimates, contracts, statements and reminders. This one belongs to the business, not to whoever pressed send: the customer reading it has never seen this software and their language has nothing to do with your menus.
Only an owner or admin can change it. It is English unless somebody changes it.
What follows neither
Anything you write or dictate yourself — an email to a customer, a note, a message — goes out in the language you wrote it in. If you tell Runny in Spanish to email a customer, the customer gets Spanish.