March 2011 at Borders
Spanish: too many
German, Japanese: 2
Arabic, Chinese, Portuguese: 1
Russian: less than 1
Korean: less than 0.5
That Borders bookstore was closed down soon after that. Recently I went to a local Barnes and Noble store and checked the foreign language books just as I did before, and the result is:
November 2015 at Barnes and Nobles
German, Japanese 0.8
Latin, Portuguese 0.5
Korean, Vietnamese 0.3
Even considering measurement errors, I can safely say that Italian has gone down, and Chinese has gone up. Russian probably went up a little and Arabic down a little.
This is of course a crude way to measure language popularity. Commercial bookstores such as Borders and Barnes and Noble make an effort to meet the demand of the market but there's no perfect, up-to-the-minute, match.
For completeness, here is another measurement of language popularity, based on an April 2014 poll of what language the language-loving people are studying conducted in the Polyglots group on Facebook, which had 16,000 members at the time. The popularity order,
French (935) > Spanish (807) > German (799) > English (651), Italian (448), ..., Mandarin+Chinese (360)
is obviously different from the bookstore popularity in 2011 or this year. For one, the stores are in Texas, where the presence of Spanish is particularly strong. Secondly, the market-determined popularity may differ from that in the Polyglots-poll because the latter is more of a reflection of the fun or leisure time enjoyment, not necessarily going along with the economic development in the region where a specific language is spoken; while studying Chinese may increase your chance to find a job in the global market, it may not be as much fun as studying the sexy Italian. It would be interesting, though, if the Polyglots group could conduct a poll every once in a while, so that we could see if the fun factor could also go up and down as the usefulness does. It probably will, but on a much smaller scale.