Like DMoney said, it really depends on the person and their relationship with the language. Since you already know French and Spanish then Latin, Italian, and Portuguese should come fairly easy to you. Sign language really is a very easy language though like someone else in the thread said. My sister took it in high school and mastered it extremely quickly.
As far as them Asian languages, I hear they are the hardest. I took Mandarin for a semester but wasn't really into it and I didn't have the motivation to put work into it so I just stopped after 1 semester. I switched to Arabic and have been taking that for 3 years now. Arabic came really easy to me and I have almost mastered the Modern Standard Arabic and am now beginning my study of Classical/Ancient Arabic. Like I said though, it really does depend on you because Arabic is considered one of the hardest languages for Americans to learn and I've seen many of my classmates try and just drop out because it was so hard. It came easy to me because although I don't speak Arabic, I am Muslim and use Arabic in my daily prayers, reading Quran, etc so everything really just fell into place.
Some good advice though is that you shouldn't just pick up a language because of how easy it is. Regardless of your background with other related languages, picking up a new one is gonna require A LOT of work initially. You should definitely pick up something you're interested in and a language that can land you a job in the future if that's what you're interested in doing.