To be fair, the real motivation for the casting was probably that studio
bosses thought they had to have big name stars on the marquee to get
people to come. They may have been right; the audience in those days
was conditioned to expect familiar faces (even if disguised). It was the
era of the studio system, with their stables of actors to choose from.
Nobody was making independent films with offbeat stories and unknown
actors.
I'm reminded of the abortive Charlie Chan TV series proposed somewhere
in the 90s. They issued a statement saying they were conducting "a
worldwide search for a Chinese actor of sufficient stature to carry the
role...who could speak English with an accent acceptable to American
audiences." This is in California, where the population includes
thousands of Asians who probably don't speak Chinese. Maybe the problem
was that they couldn't do a good fake Chinese accent.
They made the pilot with Ross Martin (known for ethnic disguises on the
Wild Wild West).
I think Keye Luke spoke excellent English. Fortunately he got a good
steady job later on Kung Fu.