Beginning this week, users with Android 6.0 Marshmallow and the Google Translate app on their phones will be able to easily translate text into their native language right inside some of their favorite apps. In-app text translation supports both app content and text that users type.
For apps that support Android's text selection methods, no extra steps by developers will be necessary in order to enable in-app translation. However, Google has outlined a method for translation support for those developers that have their own text selection behavior in their apps.
Android's in-app translation currently supports 90 languages, and will be available in wide range of apps, including selections such as WhatsApp and TripAdvisor, once it begins rolling out.
Source: Android Developer Blog