I'll admit it: For better or worse, I want to go on a 6-month (or longer) retreat in Myanmar.

For those who have done so before: Would being able to communicate in spoken Burmese be useful?

I'm good at becoming competent in languages quickly, and almost always do so before visiting a country. If it were of use to know Burmese, I'd go all in, including learning all sorts of meditation jargon. That said, I imagine the vast majority of time would be spent sitting in silence, so perhaps it's best to ration language learning time on another language.

So, would there be any benefit? Maybe in communicating better within a well-established center, or accessing more teachers? Or do the best teachers already speak english well enough to help one's retreat be as focused as possible? I've never done retreats over 10 days, so I also wonder if you just remain within the center the whole time.

Probably too late, yes, most people do not speak English. To communicate with a not English speaking teacher, you  will have way too much to learn Burmese. At Panditarama most teachers speak and understand English, still,  for them it is a very foreign language. For the female section they have at least one capable translator. Others might have them, too. Better you ask, before doing useless travelling. For daily chores, travelling in Burma, speaking Burmese is a must, as it makes things way easier and cheaper.