I will be speaking from the perspective of India.
Here, the vast majority of the rural users do not have great smartphones. With the recent release of Jio, internet speeds have improved. Still wouldn't call it great, but it is good.
Some of the most important functionality decisions that we make is offline support. We always assume that the network will not be consistent. Google Maps allows certain map locations to be saved offline. If memory serves, this was a feature that was aimed at the developing world but found great uses everywhere. Especially when hiking in locations with poor cellular connectivity.
Our apps are primarily in English. Having the ability to view the same content in the regional language is an added bonus. At the same time, we don't expect everyone who uses the app to be familiar with all the icons that are used.
The size of the application is just as important. Before the launch of Jio, data usage was capped. Back in 2012-ish(when I got my first android phone), I was using a 3G data plan which gave 130MB for 14 days. So we aim to reduce the size of the application.
Coming to design. For first time mobile and internet users, we aim to reduce the number of clicks or taps. This is just good design in my opinion.
A short tutorial goes a long way in providing a good experience.
The following blogs do a much better job in explaining app design for the next billion.
https://developers.google.com/web/billions/
https://blog.socialcops.com/engineering/android-development-...