Using multiple languages on one smartphone isn't something that's particularly uncommon, especially outside of the US. Take our Canadian neighbors as an example, they use both French and English in Canada. So what about using both languages simultaneously on their smartphones? It wasn't possible before - without having to go in and switch languages constantly. Which can be a pretty big pain. However, with Android N, it's a whole lot faster.
Google is calling the feature "Multi-Locale", and it allows the user to have a list of languages that they can reorder based on which would be the primary language and then the secondary languages. There are apps available that do support multiple languages, and those apps will now use the top choice in this list, if that language is not available in the app, then it will switch to the second one. The best part about this, is the fact that the app will do this automatically with Android N. The user won't need to do anything other than setting up their list of languages and then prioritizing them.
There doesn't currently appear to be a limit on how many languages you can set in this list. However if you do wish to set up this list (instead of just sticking with the device default that you set in the setup of your device), just head to Settings > Language & Input and at the top you will see "Languages". Just tap on that and add all of the languages you wish to add to the list. You can also drag and drop them to reorder them. If you want to remove any of these languages, simply tap the three-dot button in the upper right-hand corner, and select "Remove". You'll then see a checkbox appear next to each language. Select those you wish to delete and then hit the trash can. Pretty simple, to say the least.
This feature is going to be great for those that use multiple languages, but also those that use apps that haven't been localized everywhere. Something that isn't really popular these days, but it does happen.