Release notes - wix-billing-frontend

guides/release-notes.md

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+### Updated API: Wix Billing Backend
+
+`wix-billing-backend`
+
+We made the following changes to the [Invoices API](https://www.wix.com/velo/reference/wix-billing-backend/invoices):
+
+ + Added an optional `locale` object to [`createInvoice()`](https://www.wix.com/velo/reference/wix-billing-backend/invoices/createinvoice). It can be passed as a property of the `invoiceFields` parameter to set the locale of the customer invoice.    
+
+ + Adjusted the list of supported IETF language subtags in the `locale.language` property returned by [`getInvoice()`](https://www.wix.com/velo/reference/wix-billing-backend/invoices/getinvoice). The previous four letter language codes listed in the documentation were never returned in the `Invoice` response object. We have adjusted the listed codes to be correctly displayed as two letter language subtags.  
+
+(April 1, 2024)
+
 ### Updated API: Wix Window Frontend
 
 `wix-window-frontend`