$this->addHookMethod('Inputfield::getLanguageValue', $this, 'hookInputfieldGetLanguageValue');
$this->addHookMethod('Inputfield::setLanguageValue', $this, 'hookInputfieldGetLanguageValue');
Note that this was identified not by tracking a bug, but incidently by code reading,
while working on the request 578 "Change the default language : successful Proof-of-Concept"