Chinese, Spanish, English. That is the top 3. First due to an incomprehensible amount of speakers, second for colonialism and third thanks to globalization. Languages spread thanks to speakers. Speakers are born, they travel, interact with each other and with or without any intention, share their language with others. Languages live as long as they are used. 

This reminds me of the importance of maintaining the acquired language skill. If one already has made the effort and learned a new language, it would be a shame to lose that. Updating language skills can be fun and made with almost no effort. Read magazines, watch videos, meet new people. Anything but forget the language!

Start your day with a good morning in your mother tongue, chat a while with your international friends in language you share, write the shopping list in language you learned years ago to fresh your memory and keep the vocabulary active, watch a chapter of your favorite series in language you would like to learn and wish good night in one of the languages in danger to extinguish.

Oaidnaleapmái! (See you later in North Sami)