This is a very broad question and does, of course, depend on the extent of your knowledge of languages. If, like me, you are a native speaker of English but also study German, Dutch is a very easy language to grasp as it is very closely linked to Old English and Modern German. Dutch would, however, be tricky for a native speaker of Arabic, for example, because the two languages are on completely different branches of the language family tree.
Not only do 'nurture' factors play a role in this (how one has been educated and where one has been raised), but so do 'nature' factors; that is, a person's innate ability to pick up language. Given that language is innate in humans, everyone learns at least one language, but fewer are able to learn several due to their brains being 'more suited' to some areas (such as mathematical equations) than others (such as words or drawing or playing music etc). So essentially one could argue that the ability to learn several languages is down not so much to 'intellect', but more one's ability to learn languages.
Thus, with this combination of factors, the answer to this question would differ for every individual on the planet!