Wikipedia suggests that you download the entirety of the website on your Android device using an app called Kiwix.
The Wikimedia foundation posted to its blog today, recommending the app, which lets you download the millions of article contained within Wikipedia to your device, so you can read any article any time, even when you're offline or when you don’t want to use your data plan).
The app can also be used to download special selections of pages. Wikimedia's Renaud Gaudin writes:
In places where connectivity is a difficult (at least 30 countries on the sole African continent), the only way to access Wikipedia content is Kiwix Desktop, but it still requires a computer and electricity. Cheap Android devices might spread more quickly, and we really want to facilitate access to free content everywhere.
That said, Kiwix for Android can also be just as useful to Westerners who want to walk around with the entirety of the world’s largest encyclopedia in their pocket (if they have a big enough SD card), accessible at no cost or data fees.
You can find the app in the Google Play store.