How hreflang works
Hreflang tags tell search engines which version of a page to show to which language or region, using codes like en, en-GB or es-MX. Each version lists all versions, including itself, and every version must link back. Without return links, the tags are ignored.
x-default names the page for everyone else, often a language picker or the English version.
Questions
Is it en-uk or en-gb?
en-GB. The region is an ISO 3166 country code, and the UK's is GB.
Do I need hreflang for one language?
No. It is only for pages with translated or regional versions.
Can hreflang be in the sitemap instead?
Yes. This checker reads the HTML tags; sitemap hreflang is another valid place.
How many alternates are checked for return links?
Up to 10, to keep the check fast.