= mw.language.isSupportedLanguage('no')
false

= mw.language.isKnownLanguageTag('no')
true

Which should be wrong, as someone mentioned on Commons: https://commons.wikimedia.org/wiki/Module_talk:LangSwitch.

It seems 'no' is only a dummy alias for 'nb', which seems very strange, as 'nb' is only a individuell code.

See also:
T102533: [Bug] Disallow (or resolve) dummy language codes.
T102295: [Bug] Language fallback seems to apply even when not requested