Google accounts for 70% of web searches, Bing about another 20% and Yahoo around 7% so that leaves around 3% ( rough guess) so you are already reaching about as much of th global population as you can, do I really want all those spiders from China and Eastern Europe crawling my site.
Plus when you look at the bots they are pinging, a lot of them are Google country specific, so unless you have some form of translator it probably wont be of much help anyway.