Hopefully, with the perfection of machine translation in 30 or so years, we won't have to bother with a world-wide language.
At the moment... as much as it pains me to say it... english. Most scientific papers are written in English, as are most medical texts. It's difficult to escape the legacy of the fact that English is the language of science. Most scientists around the world speak English - they have to! To read the books, to understand the terminology.
It's also really, really, really difficult to translate some of the terms from Latin/Greek into another language. How would you translate Bilirubin Diglucuronide or glucuronyltransferase into Chinese? Or Arabic? You'd have to invent an entirely new set of words, and most scientists, even ones who only speak english as a second language, can't be bothered learning a whole new set of words to replace the English/Latin/Greek words.
Malaysia, the country I was born in, tried to change all the English scientific words to Malay words in the 50's and the 60's - this involved changing Nuclear to Nuklear. It was a completely pointless endeavour they spent millions on for the sake of 'national' pride. They abandoned it, because at the end of the day, they weren't fooling anyone by changing C to K and claiming it as a Malay word.
In a fair world, we'd all switch to either Hindi or Mandarin or Spanish. But the world isn't fair. English, from a technological, scientific, medical viewpoint makes the most sense as the world language.