English. Hands down. And will continue to do so for the forseeable future. In fact, the emergence of India and China will only further strengthen this fact. In several hundred years the "Earth Common" will still be "English", but were you or I transported to that distant time, we would not recognise it as English and would essentially have to learn this new language before we could communicate. The above trends and their results have already occured in our history with English. An English-speaking person from 2-300 years ago would be lost by what we call "English".
When China becomes the dominent cultural, economic and political force that it simply will have to become due to it's sheer size alone, they will be talking to us with English. The syntax may or may not be slightly off and there may be the occasional misuse of a word or the occasional addition of a Mandarin word, but overall it'll be 95% "English as we know it". However, as we all become used to the "new order" of having Chinese culture and interests influencing 60 to 90 percent of our daily activities, other cultures will become interested in Chinese culture (similar to how Japanese culture became so popular, but even more so) and due to this, even more Chinese words, syntax and structures will be adopted into English - more so by non-Chinese than the Chinese themselves.
All of the above mentioned activities regarding Chinese will happen to other cultures & languages (Indian, Spanish, Russian, etc.) to varying degrees depending on their degree of influence over the dominent world culture. This has been happening over and over again for the past 4-600 years and I see no existing or likely future trend that will interupt this. Over time, what we call "English" will become even more of a mishmash of other languages than it already is. After all, English is the "de facto lingua franca"!
So, why do I think it will flow this way and not simply get replaced? Because this is easier than forcing everyone to adopt a new language. Also, the Chinese, Indians, Spanish, Africans, Swedes, Russians, etc. are already adopting English as the default language to use when communicating with other cultures, whether professionally or casually. Hands up who speaks a different native tongue than what they type here at Enworld... I'm not making grandiose oracular predictions, I'm only identifying both historical precedence and existing trends. 90% of current-day "English" words ,syntax and strucures, are formed of bits and pieces of Greek, French, Latin, Spanish, Gaelic, Welsh, Scottish, Irish, German, Norse, Arabic, Yiddish, etc and has little resemblence to the old Anglo-Saxon-Norman blending that formed the original "English". At this point, adding Mandarin, Hindu, and some more Spanish to the mix is par for the course and to be expected, Heck, let's toss in some Urdu while we're at it for good measure!

Just remember, should you go time travelling into the future, make sure you stop every 50 years or so and spend about a year there to acclimatize to the changing linguistic and cultural thoughtscape. Yes, it'll greatly lengthen your travel time to the year 2206, but you'll thank me when you reach your destination and you only appear to be "quaintly old-fashioned" rather than the equivalent of a cro-mag with a lobotomy!
