EpubCheck v4.0.2 (installed via Homebrew on Mac) chokes with xml:lang attributes that have no value assigned, such as:

<i lang="" xml:lang="">blahblah</i>

by saying:

Error while parsing file 'value of attribute "lang" is invalid; must be an RFC 3066 language identifier'.

But Language tags in HTML and XML states that:

HTML and XML also provide a means to prevent inheritance of language using the empty string, ie. xml:lang="". Essentially, this says: I do not want to associate any language with this information.