On Twitter, Google’s John Mueller said that hreflang is one of the most complex aspects of SEO.
The hreflang tag is for websites with content in multiple languages. For example, a page might be available in English and German. The hreflang tag can help you to make sure that Google shows the German version of the page to German visitors.
TBH hreflang is one of the most complex aspects of SEO (if not the most complex one). Feels as easy as a meta-tag, but it gets really hard quickly.— John ☆.o(≧▽≦)o.☆ (@JohnMu) 19. Februar 2018
How to check your hreflang tags
Among many other things, the website audit tool in SEOprofiler also checks the hreflang attributes on your web pages. You can also specify an Accept-Language header in the website audit tool.