Sitemap functionality does not support locale fallback

[peterjaap]

Right now, the sitemap functionality does not fetch documents from the locale fallback if one is configured;

https://github.com/elgentos/magento2-prismicio/blob/master/Model/ItemProvider/PrismicPages.php#L73

To fix this, we need to run two queries; one for the original locale, followed by the fallback locale but excluding the already found document ID's using the first query.

Maybe we can use \Elgentos\PrismicIO\Model\Api::getDocumentByUid