How to get the order locale on checkout using the Admin API

[Tommytks]

Hi all - Does anyone know how to get the order locale used by customer when he/she checkout at the page from the Admin API response? we are constructing an App with webhooks registered to listen to order create/fulfilment, and we couldn't find a suitable attribute from the Shopify Order API in both graphQL and restful Response for locale. The custom_locale seems not to be related to the locale used on checkout. Appreciate if you can help!