It’s hard to see the downside of everyone speaking Russian or German when compared with the current situation, but English would be better because 1) It would require the fewest people to learn a new language (sort by total speakers in the link above) 2) would be best for communicating with visitors and 3) fuck Russia
Obviously it’s easy to assume that because China has the most people, Chinese must be the most common language. But when you look at total people who know a language (as first, second, etc.) the most spoken language in the world is…English, with about 400 million more speakers than Chinese.