It wouldn't be that hard to find good actors from specific nationalities, it would just be very time and money consuming to cast such a wide net.
But a lot of big series right now are casting actors from outside the UK or USA, they just need to have an agent in that country in order to have a foot in the door. You can bet that the main cast will be newcomers/unknown actors, since they're way less expensive, with a few big names attached to the project for a bit of star power. Shanks will probably be a famous actor and a couple of the important strawhat mentors, like Bellemere, will also warant someone with weight and skill behind their name.
The problem isn't necessarily finding actors from different countries, that part is actually is pretty common, and there will be no problem if that happened. What's quite difficult here is assigning certain roles based on nationalities first. For example, finding a Russian actress for Robin that happens to speak English fluently, have the talent, and fit the role better than other non-Russian candidates is almost impossible.
Also, why would Oda be thinking about specific actors when he made those SBS jokes? He clearly just went with basic stereotypes.
Luffy's always in shorts, flip flops and likes parties -> brazilian. Sanji's french because of the cuisine, Franky's from the US because he's loud and likes cola, Brook is austrian because of the classical music scene (Mozart and Haydn where born there, Beethoven spent a lot of his life there), etc.
I do agree with you here. I was just trying to figure out what quote from Oda was misinterpreted.
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