I am most concerned with Book or Event schema types which both use inLanguage. I have a number of books that are in 2 languages, the clearest example would be a Chinese-English/English-Chinese dictionary.
The inLanguage blurb has
The language of the content or performance or used in an action. Please use one of the language codes from the IETF BCP 47 standard. See also availableLanguage. Supersedes language.
but if you drop in an availableLanguage list you get
warningavailableLanguage
http://schema.org/#__sid=rd7 (The property availableLanguage is not recognized by the schema (e.g. schema.org) for an object of type Event.)
Have I missed something other than "pick one"?