Xiaomi is currently the number one smartphone manufacturer in China and this is all without giving customers access to the Play Store on their devices within the country. Instead they have their own app store in place to offer a wide variety of apps. Xiaomi is continuing to expand within the Chinese market and all the while gaining more market share, and they're pursuing plans to expand out into other markets to offer their handsets to consumers in other regions. Even though they're looking to expand though and bring their phones to people outside of China, according to Hugo Barra they won't be trying to do the same thing with their app store. That is to say the Xiaomi app store is not planned to leave outside the Chinese market.
The basic reason behind this is because Xiaomi is a Google Services partner and according to Barra Xiaomi has been a partner since the first time they sold handsets outside of their home market. This means that Xiaomi works closely with Google in other markets where they currently sell Xiaomi handsets, and will be doing the same thing when they eventually(hopefully)expand into larger markets like the U.S. With no Xiaomi app store in place on Xiaomi phones, users won't be left without a place to download and install all of their apps, instead they'll have access to the Google Play Store like any other Android handset would who is also a Google Services partner.
Barra not only iterates that they will continue to work closely with Google to offer their mobile services on Xiaomi's handsets in other markets, but he also believes that Google Mobile Services are a necessity for essentially any other market, which is also why they won't be looking to expand their own app store outside of the region. Barra also mentions that while they would love to bring their devices to the U.S. including their top flagship products, it isn't anywhere near the top of Xiaomi's priority list. Instead, right now they're focusing on other overseas markets like Russia, and Indonesia, and India which he states is their biggest international market at the time.