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So if a random person from anywhere in the world were to be teleported to my location, there's a 25% chance I would be able to understand them. ... And that chance would double if I learn Mandarin.
Only if they only spoke one language. Googling indicates there are somewhere around 1.45 to 2 billion total English speakers, so just knowing English might hit 25% already.
Edit: Also, the graph only lists languages with 50 million speakers, so the real proportions are smaller.
If you teleport to a random location there is a pretty good chance of sinking immediately into the ocean never to return so hope you speak fish? Following that are the inhospitable locations like deserts, deep in some forest or other, frozen mountains and other slightly slower routes to your eventual demise. Then all the places where you will be regarded as some sort of invader or trespasser.
Looks like about 70% is later and only about 10% of the remainder is inhabited so about 3% chance of landing somewhere with land and people.
http://www.curiousmeerkat.co.uk/questions/much-land-earth-inhabited/