Hopefully, you have already read (and used!) “How To Say ‘My Friends Are Nice’ in 42 different languages.
That post was originally meant to contain ten languages. However, finding no ready-made lists, it seemed easier to list all available translations, rather than to decide on only ten languages to list. There are several ways to determine the top ten languages. We might list them based on numbers of native speakers, or total numbers of speakers, or languages spoken in the most countries. Some even rank the importance of a language based on the potential economic impact of that language.
More detailed information follows this summary chart:
Top 10 World Languages:
(c)Tardyslip.net 2009
Wikipedia has a list that ranks languages by the number of native-language speakers, while including some data for second languages. The top languages, in this kind of ranking, are:
Ethnologue Part I, “Languages of the World,” tabulates and compares world languages and number of speakers, leaving us with this top ten:
“It turns out that 389 (or nearly 6%) of the world’s languages have at least one million speakers and account for 94% of the world’s population. By contrast, the remaining 94% of languages are spoken by only 6% of the world’s people.”
Part of the Ethnologue, 16th Edition, M. Paul Lewis, Editor. Copyright © 2009, SIL International. All rights reserved. http://www.ethnologue.com/ethno_docs/distribution.asp?by=size

http://www.ethnologue.com
The Missouri State Department of World Languages explains their ranking of the top twenty languages:
A simple first question we might ask is: How many native speakers are there for each language? Estimates of the number of native speakers for the worlds “top twenty” languages are provided in The Cambridge Encyclopedia of Language (2nd Edition, Edited by David Crystal, Cambridge University Press, 1997) and The Atlas of Languages (Edited by B. Comrie, S. Matthews & M. Polanyi, New York: Facts on File, 1996).

http://vlc.missouristate.edu/dol/worldlanguages.asp
Steinke Language Index from the Steinke Institute in Bonn, Germany, ranks languages according to their economic strength.
“These rankings of course, say nothing about the value of a language, but only reflects the potential professional and economic benefits, which we can draw from their learning can be.”
Note: All links to information on this page was accessed/accessible on July 16, 2009.