Just because it's spoken by more native speakers than any other language that doesn't automatically make Mandarin more relevant in a global context.

English is a global language due to its history (largely grounded in the British Empire's colonial history). Mandarin on the other hand in a global context as of today is still largely irrelevant because it's only spoken in one country, even as that country is as globally relevant as China.

That might change in the future but unless you specifically deal with China and Chinese businesses Mandarin is no more relevant on a global level than - say - Russian or German.

One might think so at first glance, but it's much harder to learn (friction reduces adoption) and quite uncommon outside Asia, so it's only useful in specific situations. Plus it's not the official language of any European country.

Mandarin? I would not mind that as well, but that is even more extreme than the other suggestions. It would get no votes.

English is mandatory in Chinese education, from primary school to college.

Chinese is not mandatory anywhere in Europe.