A language you are interested in.
In 8th grade I started taking Spanish. Some of my friends took it with me, some took German, others took French. We all got to the same levels of mastery, except of course for my Mexican and Cuban friends who took Spanish and already spoke it at home, of course. (actually on some levels it was harder for them, as they spoke dialects, not textbook Spanish, so they were RElearning which can be even harder)
I later took Japanese and it was just as simple as Spanish for me (not that it was simple...I could have said "just as difficult"...the point is that it was "just as _______ for me").
Now I keep trying to get someone to teach me Korean, so I can communicate with my MIL better, but I'm not TRULY interested in doing that, so I'm finding it difficult to teach myself (and can't find a teacher).
So I say there's no true answer to that.
I mean, some people find it impossible to learn ANY second languages. Others never become fluent in a second language (MIL has lived her 40 years and is not anywhere near fluent, though she's a heck of a lot better in English than I am in Korean, or currently am in Spanish or Japanese!).
While others pick them up easily and quickly.
The brain varies, and I don't think there's any one answer.