I am building a beach house on the Caribbean coast of Colombia. The lumber available is exotic hardwood with an actual measurement of 4" x 6". Will floor joists span 12' on 16" centers with less than 1/2" deflection?
If it is decent, yes.
Welcome to the forum Doug!
Without knowing the species and mechanical properties it's a bit of a guess as to whether the 4x6 on that spacing will meet your needs. If it is one of the species tested in this analysis, you're probably good to go:
https://www.fpl.fs.fed.us/documnts/fplrn/fplrn242.pdf
There is a wonderful beam calculator that Don_P has put up on the web at http://www.timbertoolbox.com/Calcs/beamcalc.htm
If you plug in some appropriate numbers in that calculator you'll discover that a true 4x6 of any given species and grade is a little stronger than a single 2x10, which measures 1.5 x 9.25. The 4x6 is a little more flexible though, so will result in slightly more deflection than a 2x10. If a 2x10 works in a particular span then your 4x6 will too, although may be a little bouncy if it's close to the max span in the table.