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.