The easiest of all those questions to answer would be will everyone know English, quite possibly. Though with Europe gaining a stronger presence in the world though slowly I'm sure they will slow.
hmm, Greek was once the common language because of their (seafaring) trading & advancements had come into common usage. While English has become a common language aboard due to trade, we see many areas in the USA have bilingual access in Spanish.
Education seems incline toward bilingual or multiple languages such things may produce a common language as terms from different languages come into common use.
Will Westernization and McDonalidization erode traditions?
Interesting use "McDonaldization", but then the supposed Christian => democracy accepted China's allowing capitalism into mainland China as an indication of China's acceptance of the American Way => Christian democracy!