MediaWiki defines some "dummy" language codes as aliases for other "real" language codes. These are defined in the $wgDummyLanguageCodes variable.
Wikibase currently treats such "dummy" codes as real language codes in some contexts, while resolving them to the "real" code in other places, leading to confusion such as T102295, where there are conflicting labels for the "no" and the "nb" codes, even though "no" is an alias for "nb".
Wikibase should either disallow such codes completely in all API input, or should resolve them to real codes. Resolving them is convenient for compatibility, but leads to unclear semantics when both codes are used. Disallowing them breaks compatibility with existing data.