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Has the content of the pages been updated? Could this be direct result of query mismatches now vs previous content in Spanish and Portuguese ( i suppose those are the content languages used in Latin America). Have you made sure no 3rd party coincidences are not causing this (competitor coming along strong in the same period of time, market demand changes, search phrases changes)? Have you compared your organic arrival queries from before to now and see if you are getting similar numbers of hits for specific key phrases? There has got to be some further digging involved here, this just sounds way too odd to be a natural situation, something has got to be funny or wrong.

You can submit them separately. One for video, one for images, one for URL's. This may be a more effective approach at getting things indexed, as it separates them into their own category. If you are already having a high load time, wouldn't hurt to try. To answer your original question: "Sitemaps should be no larger than 10MB (10,485,760 bytes) and can contain a maximum of 50,000 URLs. These limits help to ensure that your web server does not get bogged down serving very large files." But wait, there's more! http://www.seroundtable.com/archives/021559.html "Google has changed the number of Sitemaps you can reference in a Sitemap index file. The number use to be 1,000 sitemaps can be referenced in a Sitemap index file, now the number is 50,000 Sitemaps. This is a huge increase in capacity. Still, each Sitemap file can contain up to 50,000 URLs, so technically 50,000 multiplied by 50,000 is 2,500,000,000 or 2.5 billion URLs can be submitted to Google via Sitemaps."

Thanks for the hint. Basically I'd always use the local language for the Urls, but I wonder if the japanese symbols are in any kind problematic to read for the search engines.

Happy to help! Let me know how  it works out.

Just in case anyone stumbles across this topic: We started using honeypot captchas in 2011 and it really paid off. Not only because we got rid of the old captchas, but also because they are keeping out 99,99% of all bot inquiries or spam.