Prior to 1850 the diplomatic language (the language used by governments around the world to communicate with each other) was French.  But, after 1850 it had changed to English.  The Kings after 1850 had traveled extensively around the world.  They knew that Polynesian was spoken no were else in the world.  They also knew by first hand experience that you could order a cup of coffee in Germany using English, but not in Polynesian-Hawaiian.