You can change your default language in the 'Basic Info' tab in the Settings->Profile menu. This will change the language of your administrative portal.
You can also assign custom languages to each customer. To do this, select a customer to edit under the Customers menu, and on the "Editing customer" page, choose a language for that customer in the "Language" field drop-down menu. In the example below, 'V for Vintage' will see the store in English.
You can set a supplier's default language in the 'Supplier Details' tab of the Suppliers menu.