most useful languages?
In your opinions, what are the 10 most useful languages you can learn on Duolingo?
I wish you were more specific on what you want to use a language for, or for what area(s) of the world. But let's assume you're in the West and you're asking about most useful for career opportunities. Keep in mind that Westerners speaking rarer or more exotic languages to them are valuable in some business areas.
Spanish, Portuguese, Japanese, Korean, German, Vietnamese, Russian, Hindi, Mandarin, and Indonesian, though the last three are not complete yet. Arabic is a good one too, but not on Duolingo for English speakers.