Language codes search engines don't recognise

hreflang tells search engines which language, and optionally which country, each version of a page is for, so a visitor in Germany is shown the German page. Google reads only real codes: "Only language codes listed in ISO 639-1 and region codes listed in ISO 3166-1 Alpha 2 are supported; other codes that aren't listed in those standards, such as es-419, aren't supported." A code it can't read is a version it doesn't know about, and the wrong one turns up in that country's results.

The usual mistakes are a country where the language goes (jp for Japanese, which is ja), en-UK (the United Kingdom's code is GB: "using EU, UN, or UK in hreflang annotations doesn't have an effect on Google Search"), and an underscore (en_GB). Google also warns: "You can't specify the country code by itself." Some country codes are real languages too (uk is Ukrainian, se Northern Sami), and those are taken at their word. A language with no two-letter code at all (Cebuano, Filipino, Cantonese) can't be named to Google, which is nobody's mistake, so that is only a notice.

Whether each version links back to the others is a question about the whole site, which indexing.hreflang-pairs answers for monitored sites. This rule reads the codes in the page's HTML, not in a Link header or the sitemap.

Sources: Google's supported language and region codes and common hreflang mistakes.

A language from ISO 639-1, then optionally a region from ISO 3166-1, joined by a hyphen:

<link rel="alternate" hreflang="en-GB" href="https://www.example.com/cakes" />
<link rel="alternate" hreflang="ja" href="https://www.example.com/ja/cakes" />
<link rel="alternate" hreflang="zh-Hant" href="https://www.example.com/zh-hant/cakes" />
<link rel="alternate" hreflang="x-default" href="https://www.example.com/cakes" />

Translation plugins (WPML, Polylang, Weglot) take the codes from each language's settings, so a wrong code is usually fixed in one place there.

A language with no two-letter code can't be given one. Its version can stay for the people who pick it, and an x-default version (a language chooser, say) is where Google sends readers whose language isn't listed.