I'm a Systems Reliability and DevOps engineer for Netdata Inc. When not working, I enjoy studying linguistics and history, playing video games, and cooking all kinds of international cuisine.
English (because we have more second language speakers than any other language in the world)
Mandarin Chinese (has more native speakers than any other language in the world)
Hindi (mostly because of the very high population in India and the fact that it was a common intermediary language there before English became big)
Spanish (due to the spread of the language, not centrally high populations)
French (due to second language speakers)
Standard Arabic
Bengali
Russian
Portuguese (mostly because of Brazil)
Indonesian (similar case to Hindi, just in Indonesia not India)
Of these, if you already speak English, Spanish is probably the easiest to learn. Learning it also makes other Romance Languages (Portuguese, Italian, French, Romanian, etc) easier to learn, and it's close enough to Portuguese that you can get your basic point across pretty easily if you need to.
French is a decent choice also because it's a widely used diplomatic language (and also one of the main languages for a lot of international organizations), but it's harder to branch out from than Spanish because it's very much the odd-one-out among the Romance languages.
If you don't mind a challenge, Mandarin Chinese is almost certainly your best option though, as most people who speak it do not speak more than a few basic words of English. Just understand that it is seriously difficult coming from English (many semantic aspects of the language are drastically different from English), and that the likelihood of finding other people who speak it outside of China or areas with lots of Chinese immigrants is very low.
All of the rest are not as likely to be encountered outside of the regions where they're an official or administrative language, and you're pretty likely even then to only end up dealing with people who also speak English (or possibly Spanish in the case of Portuguese) unless you choose to live in such a region.
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For quick reference, the most spoken languages in the world, based on data from Ethnologue, starts with:
Of these, if you already speak English, Spanish is probably the easiest to learn. Learning it also makes other Romance Languages (Portuguese, Italian, French, Romanian, etc) easier to learn, and it's close enough to Portuguese that you can get your basic point across pretty easily if you need to.
French is a decent choice also because it's a widely used diplomatic language (and also one of the main languages for a lot of international organizations), but it's harder to branch out from than Spanish because it's very much the odd-one-out among the Romance languages.
If you don't mind a challenge, Mandarin Chinese is almost certainly your best option though, as most people who speak it do not speak more than a few basic words of English. Just understand that it is seriously difficult coming from English (many semantic aspects of the language are drastically different from English), and that the likelihood of finding other people who speak it outside of China or areas with lots of Chinese immigrants is very low.
All of the rest are not as likely to be encountered outside of the regions where they're an official or administrative language, and you're pretty likely even then to only end up dealing with people who also speak English (or possibly Spanish in the case of Portuguese) unless you choose to live in such a region.