Preliminary thoughts on this is that a new data type for Language be constructed, along the lines of the data type Date, with reference to a standard.

Which standard?

For the one class where Google specifies a standard it is IETF BCP 47
In Yandex documentation the language code specified is ISO-639 (while it's not stated explicitly in the documentation, the code type employed in their examples is ISO 639-1)
Bing still recommends use of <meta http-equiv="content-language"> for language declarations, where the value for content is "comprised of a 2-letter ISO 639 language code, followed by a dash and the appropriate ISO 3166 geography code"
JSON-LD specifies BCP 47 for @language values
In their specifications for implementing the hreflang tag both Google and Yandex stipulate ISO 639-1 for the language, and ISO 3166-1 alpha-2 for the optional region declaration (appended to the language code with a dash as separator)
W3C's HTML5 recommendation on language calls for a valid BCP 47 language tag
W3C's internationalization documentation on "Choosing a Language Tag" and "Language tags in HTML and XML" both cite BCP 47
schema.org/inLanguage, of course, expects the value to be Language or Text, with the directive "Please use one of the language codes from the IETF BCP 47 standard"

All things being equal it seems to me that BCP 47 should be the standard for schema.org language declarations.

All of this glosses over the data model issue that Language is already constituted as [a class in schema.org](Please use one of the language codes from the IETF BCP 47 standard), rather than a data type - and perhaps there's reason why the data model would want to support Text as an expected value rather than only pointing to BCP 47 (although this isn't done for Date, although dates, like languages, can also be represented in content as a string).

Regarding the comment by @mfhepp, aside from the response from @danbri I'd point to the potential limitations of language declarations in RDFa and JSON-LD in the context of possibly extending the classes on which inLanguage (or a similar property) can be used, as per #1065 started by @betehess. I.e. JSON-LD permits @language only to be declared for a string, not a URI.

Language type lacks guidance on properties #1079

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