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No clear winners here, but only slight advantage to Draconic (generally I have magic books and stuff written in this for tradition) and Orcish & Goblinoid because they are pretty much in all my campaigns.
Elves are quite common with my group so elven is the most popular non-human language. Since I use the human language variant from SCAG, the next two most common languages are Oeridian and Flan.
I tend to use lots of goblinoids and fae so goblin and sylvan are good options. I also feature gnomes and giants often and one campaign area is a lot of islands harrassed by Sahuagin (who speak Aquan, the language of the Whales).