rutger5000 said:
Purely practical: German (it's the language of eastern Europe) Latin (then you can sort of figure out French,Italian, Spanish and Portuguese), and Chinglish. For roughly a bilioen people are going to think they speak English, but they really won't.
Latin is certainly helpful when learning French, Italian, Spanish and Portugese, but I don't know about the "figure out" part. I've studied Latin, French, Italian and Spanish, but Portugese still sounds like Spanish being spoken by a Polish person. Underwater.
Brazilian Portugese is a bit different and seems to have other languages mixed in... The instructor at my BJJ school keeps saying something which sounds like "ashpet" (meaning stop) but Iberian Portugese speakers I know don't understand this. In Italian, however, "aspetta" means "wait", so I'm guessing there's a link to that, possibly due to large numbers of Italians in Brazil.
I'd say that Latin was most directly helpful for learning Italian. The fact that one was the spoken language whilst one was the written language (Dante Aligheri was the 1st author to write in Italian) means a lot of Italian is just simplified/corrupted Latin. For instance, the Italian word for "really" or "indeed" is "davvero". The Latin equivalent is "ita vero".
In addition to brushing up the languages I already speak I'd like to get to grips with Farsi, Arabic, Greek and Russian. I've got phrasebooks and TYS packs for Tagalog and Hebrew as well, but I doubt those are gonna be super-useful any time soon. I did a year of German in school but again, not going there anytime soon and most of them speak good English.