The additions are to the T-Mobile ONE plan and the carrier's Simple Global roaming package, which as of July 22 will cover "over 210 countries and destinations" according to the carrier.
The big four United States carriers (possibly soon to be the big three) are always arguing about who has the best network, but that's usually just marketing spin.
Before you go celebrating the new coverage areas, it's important to note that in these countries, customers will not get access to free high-speed 4G LTE data, but rather unlimited data at 128kbps (aka 2G). Making phone calls internationally on T-Mobile will cost 25 cents a minute.
The carrier is also adding a new option to get faster speeds overseas. Pay $5 and get half a GB that can be used in a single day and you can make as many phone calls as you want within 24 hours of purchasing the roaming pass.
As it stands now, T-Mobile has the best pricing for these data passes and the most countries covered. Still, if you're making a shorter trip, don't have an unlocked phone, or just can't deal with the potential language hassle of buying a SIM, T-Mobile Simple Global certainly covers more than AT&T or Verizon's worldwide data passes, and its high-speed add-on is half the price, too. You can, if you prefer, purchase a second data pass, getting another 512 MB of LTE-speed data for another $5.