Invalid language or region codes
IssueHigh priority
Pages with an alternate whose code is not a language, or not a region - en-UK rather than en-GB, for instance.
Why it matters
An alternate with a code search engines do not recognise is ignored.
How to fix it
Use an ISO 639-1 language, optionally followed by an ISO 3166-1 region: en, en-GB, pt-BR. Use x-default for the fallback.
Skitter runs this check on every crawl, and lists each page it finds with the evidence: the header, the tag or the link that gave it away.