dc:language epub - SERP Analysis

https://kb.daisy.org/publishing/docs/epub/language.html

https://www.w3.org/TR/epub-33/

https://www.mobileread.com/forums/showthread.php%3Ft%3D87928

https://www.reddit.com/r/selfpublish/comments/1dyko3j/epub_help_xhtml_xml_css_dctitle_unbound_error/

https://standardebooks.org/manual/1.6.1/9-metadata

https://forum.aspose.com/t/set-language-of-an-epub-file/251376

https://community.adobe.com/t5/indesign/epub-displaying-right-to-left/m-p/9572442

https://www.amazonforum.com/s/question/0D5at00000OXGY4CAP/send-to-kindle-does-not-respect-full-language-tag-in-epub

https://github.com/w3c/epubcheck/issues/702

https://w3c.github.io/epub-specs/epub33/multi-rend/