Stumbleupon will soon be in Spanish
According to reports advanced users who live in non-English speaking countries, SU is working on the translation of the interface.
But given the nature of SU, not only besides translation but also the way to get users will not accustomed to social bookmarking sites, at least for Spanish speakers.
In my opinion the International Stumbleupon launch will take place before year end.
Hope you have success and achieve capture how users encountered and Latin America.