It should return false.
Note that the underlying error may be in Language::isKnownLanguageTag.
Here's a test page: https://test.wikipedia.org/wiki/Foo%27s_bar
And here's the module: https://test.wikipedia.org/wiki/Module:IsKnownLanguageTag
You can just test this with mw.language.isKnownLanguageTag("'") or something like that.
Obviously, it should return false -- it's not a known language code!
Version: unspecified
Severity: minor
URL: https://test.wikipedia.org/wiki/Foo%27s_bar