7.2. The Language Pseudo-class: :lang()
https://www.w3.org/TR/2026/WD-selectors-4-20260122/#the-lang-pseudo

Note: It is recommended that documents and protocols indicate language using codes from [BCP47] or its successor, and in the case of XML-based formats, by means of xml:lang attributes. [XML10] See “FAQ: Two-letter or three-letter language codes.”

The BCP number obviates the need for "or its successor" (its the point of using the BCP number)

The word "codes" should always be replaced with "language tags". BCP47 grokketh not "codes", only "tags" and "subtags". (The underlying ISO standards have "codes").

The link to the FAQ is really going the wrong place. You should probably link to our article on choosing language tags (https://www.w3.org/International/questions/qa-choosing-language-tags) instead.