I think learning a second language in school has enough ancillary benefits to make it mandatory, but there is no language other than English that is so widespread to make that particular langauage dominant.  I think that within the limits of the resources, schools should try to offer a few choices and let the students choose.<p>Sometimes a language will appeal to a student because something they are interested in is heavily tied to that language.  A serious student of judo would likely benefit from speaking Japanesse if only to be able to talk with other practitioners.  A serious student of Go may be interested a language from a country where that is common such as Japanese or Korean, etc.  On the other hand, someone interested in philosophy is likely to benefit from French more than most other languages.<p>If we absolutely had to pick one, I would focus on Mandarin Chinese.  China is a rapidly rising power that is becoming ever more important in the international scene and houses a huge percentage of the world population.