When should you use the hreflang tag in Sitemaps?

It generally is not necessary to put hreflang tags in sitemaps. They can be put in within pages or within headers. However, the use in sitemaps may be appropriate for fully multi-lingual versions of the same content.

Use hreflang or sitemaps for language- or region-specific pages. If you have multiple versions of a page for different languages or regions, tell Google about this use hreflang.