For a great resource on language, both in terms of geographic spread of a given language as well as the languages and dialect spoken within a country (as well as on language status (official, etc.) and number of speakers), see:
http://www.ethnologue.com
(Then you may wish to go to "Search the web version" to find the on-line resources.)
There is also the excellent resource which provides detailed information on geographic distribution of language (by language or country):
http://www.rosettaproject.org/
For one organization's per-country-assessment on the language situation, see:
http://us-english.org/foundation/research/olp/
For a breakdown of language by country, see:
http://www.infoplease.com/ipa/A0855611.html
Describes the stature of English's spread:
http://www.theatlantic.com/cgi-bin/o/issues/2000/11/wallraff.htm
The following are links to sites of interest on Esperanto, and are relevant not only for consideration of an invented language for the ultimate choice, but also may be of interest for those interested in issues related to the political adoption of any language as a world language:
http://www.esperanto.net/veb/faq-9.html
http://ttt.esperanto.org/us/USEJ/world/kontraux.html
http://infoweb.magi.com/~mfettes/psyres.html
For an assorted list of resources on international auxiliary languages, see http://www.geocities.com/Athens/Forum/5037/
Although our World Auxiliary Language Campaign is strictly neutral as to which langage should be chosen as the eventual world auxiliary language (leaving it up to our representatives in consultation with scholars in various disciplines to decide--those who have the authority and capacity to put such a language into effect through mandated instruction in all public schools at an early age throughout the world), some of our visitors may be interested in expressing their views as to which language they feel would be the ideal candidate. To do so, the website at http://freewebs.com/international-languages/ is soliciting such information.
(Feel free to submit any links for possible inclusion here if they may be of concern to those interested in a world language (particularly but not exclusively if the link is to a group not dead-set on one language or kind of language). Submit to webmaster)