Voted for Spanish 🇪🇸.
Spanish is the 4th most spoken language in the world and 2nd as a first language only to Chinese.
Here are your options in order of approximate total speakers...
Spanish — 500 million, 4th most spoken
French — 300 million, 5th most spoken
Russian — 300 million, 7th most spoken
German — 100 million, 12th most spoken
Korean — 80 million, 20th most spoken
Btw, I bring these figures up because while you probably already know which is the next most common for users of youdApp these speak to the audience growth potential among (1) non-english/chinese speaking users who don't yet use your app and (2) those in the same subset who don't yet use distributed apps or cryptocurrencies generally.