Important: Make sure that your provided hreflang value is actually valid. Take special care in regard to the two most common mistakes:
Missing confirmation links: If page A links to page B, page B must link back to page A. If this is not the case for all pages that use hreflang annotations, those annotations may be ignored or not interpreted correctly.
Incorrect language codes: Make sure that all language codes you use identify the language (in ISO 639-1 format) and optionally the region (in ISO 3166-1 Alpha 2 format) of an alternate URL. Specifying the region alone is not valid.