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phaeberle - 1:19 am on Aug 2, 2007 (gmt 0)
"Half the money I spend on advertising is wasted; the trouble is I don't know which half."
Your best shot would be by IP. You can get an IP database and once somebody clicks on an ad you could compare it to your IP database and depending on how big the spanish speaking comunity is you can decide what language you show.
You can learn from the US Census how big a spanish comunity is in regards to a Zip code.
However always keep in mind the quotation below
John Wanamaker