All it takes to avoid that is looking into commit histories of projects in each language and figuring out how they mark bugfixes. If it's suspiciously reliable

All it takes to avoid that is looking into commit histories of projects in each language and figuring out how they mark bugfixes. If it's suspiciously reliable you would know you made a mistake classifying bugfixes somewhere or your sample is too small or the programming practice in the language is unusual and needs closer look.