By default checkout page is displayed in the language defined in the following priority
1. browser cookies
2. parameter lang sent via API
3. language preferences in a browser
If a customer has already chosen a preferred language, visiting checkout page before, it will be stored in a browser cookie named lang.
If the cookie is empty, but the merchant used parameter lang when creating order, in this case, the checkout will be displayed in the language from API parameter. See more details
If the cookie is empty and parameter lang is not used, the checkout will be displayed in the language of browser preferences. See more details: https://www.w3.org/International/questions/qa-lang-priorities.en