Six regions already identified by conservation scientists as hotspots -- Mexico to Panama; Colombia; Ecuador to Peru; Paraguay and Chile southward; southern Africa; and Australia -- were estimated . . . to contain 70 percent of all predicted missing species.
From here, citing Lucas N. Joppa, David L. Roberts, Norman Myers, Stuart L. Pimm. Biodiversity hotspots house most undiscovered plant species. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1109389108.
I'm somewhat surprised that such a large swath of Latin America figures so prominently, or that southern Africa and Australia, which have had a British colonial presence for so long would still be expected to have so many undiscovered species. I would have expected more undiscovered species in Southeast Asia and the Congo basin.