To set your default language that will change ALL checkout pages/widgets across your entire account, access the Configure menu and click Platform Settings.
Then select your Default Language.

Campaign Default:

If you would like to override the language settings for a specific Campaign, you can change them on the Edit Campaign settings page, under Communication.

Specific product language override:

Vendors can also set specific products to specific languages in the product settings. This is useful if you are selling products in a customer campaign and need to create multiple products in different languages.

Please note that some text may not automatically translate, because it is vendor-controlled text like the Legal Notice option, Sidebar Content, etc.