If you could instantly be granted fluency in 5 languages—not taking away your existing language proficiency in any way, solely a gain—which 5 would you choose?
Mine are German, Spanish, Japanese, Russian, and Vietnamese (so I can understand what the ladies in the nail salon are gossiping about).
Already speak Spanish and am learning Vietnamese for the very reason you mention, but also to respect my nail tech. She's very nice and we have fun teaching each other new words (Vietnamese is difficult because of the tones and my lazy Southern tongue would prefer to drawl) so I'd go for Russian, Japanese, Chinese, German and French.
Arabic & latin – because so many ancient Greek documents were translated in the Middle Ages to these two languages.
Mandarin – because it actually sounds pleasant.
Hindi – Access to ancient writings
Russian – because it rolls the letter [R] and I can already do that because I speak Spanish.
Spanish, O'odham, Greek, Italian, German
Spanish, Mandarin, Russian, Gaelic, Dutch.
Spanish, Mandarin, and Russian for their ubiquity, Gaelic and Dutch because I think they'd be fun to know.
French, Spanish, Hebrew, German, Italian
French, German, Greek, Farsi, and Hindi.