Why would it be better RP?

Well, you knew the language of the country you are playing means you are better at researching and behaving like it. ^^

Not at all necessarily. In truth, there's hardly any corelation. I bet most English-speakers can't even know when Guyana or Sudan got independence, just since they speak the tongue doesn't make them smart about the homeland of the tongue. Invite history nerds if you want that.

Hardly any correlation? While I would agree that would apply to countries that have adopted their ex-colonizers as their official language, I would argue that there is definitely a benefit for people RPing places where their native tongue is part of their nationality.

What is the benefit, then?