As noted in w3c/epubcheck#702, we're not completely clear about whether dc:language has to be well-formed or valid to bcp47 (sounds like it could be either right now):
The metadata section MUST include at least one language element with a value conforming to [BCP47].
https://www.w3.org/publishing/epub/epub-packages.html#sec-opf-dclanguage
Having had this discussion in web pub/pub manifest and ultimately coming to the conclusion that enforcing validity doesn't add a lot of value, I'd suggest we clarify the same requirement applies here. It's all epubcheck has ever enforced, so it won't affect existing content.
As noted in w3c/epubcheck#702, we're not completely clear about whether
dc:languagehas to be well-formed or valid to bcp47 (sounds like it could be either right now):https://www.w3.org/publishing/epub/epub-packages.html#sec-opf-dclanguage
Having had this discussion in web pub/pub manifest and ultimately coming to the conclusion that enforcing validity doesn't add a lot of value, I'd suggest we clarify the same requirement applies here. It's all epubcheck has ever enforced, so it won't affect existing content.