Myanmar was actually surprisingly good place to travel for English speakers. In Singapore where I live I primarily communicate in English and I found it relatively easy to speak English to get around Myanmar as well. In hindsight this makes sense since Myanmar was also a British colony for a long time
This was especially true in Yangon and Bagan. In Yangon I found that the English level of pretty much everyone I had contact with to be great, from hotel staff to taxi drivers to airport/train station staff, to shop owners. In Bagan it was the same experience.
Mandalay was pretty decent too. This is not completely objective but I felt like the proficiency wasn't quite as high as Bagan and Yangon. When I was at the night market I couldn't really speak English with anyone there. Although to be fair a night market in Yangon would probably be the same
I think outside of these 3 major cities, English proficiency is actually quite low in the countryside. So if you plan to venture outside these urban areas, you should learn more Burmese