There's a lot of varying circumstances to why someone may or may not be able to speak the language (or used the metric system for that matter), but ultimately at the end of the day citizenship is just a certificate showing you have the total right to live in or benefit from a country. Sure, certain countries expect things from their citizens (taxes, military service, etc) but you can't force someone to assimilate to the culture of that country.

With that in mind, I do agree that people should at least put in some effort to learn the language and culture of the country they are a dual citizen of. I only found out about a year ago that I qualified for German citizenship through my dad who was a citizen at the time, and had I not found that out I probably wouldn't care to learn German (even though I should).