The translator is based on the Bergamot project (https://browser.mt/) and making it work on Android involves installing their Firefox extension, which used to be the way the feature was implemented before it got merged natively into the browser on desktop.

You should make sure that you have the Firefox Beta, Nightly or Fennec installed (that shouldn’t be necessary once Firefox brings extended add-ons support to stable Android releases too hopefully later this month).

The first step is to enable the debug menu by going to “Settings” -> “About Fennec/Firefox” and tapping a few times on the app icon until you see debug menu enabled.

You then have to go back to the “Settings” menu and go to “Custom Add-on collection”. Here you don’t need to create your own collection on Mozilla’s website, you simply need to type in the id and name from Firefox Translation’s Github page: https://github.com/mozilla/firefox-translations (see the “Android” section).

Once you do that, go to the add-ons manager and you’ll see the Firefox Translation extension there and now you simply have to install it.

The extension works in two ways: you can translate whole web pages by taping on its icon in the bottom right of the screen, or you can translate custom inputs by opening the extension from the add-ons menu. All of that is done offline by your phone’s chip (I’m guessing it’s only using the CPU since on the desktop version they talked about SIMD code and no mention was made of GPUs) and it’s really fast.

The only caveat is that the extension is not maintained anymore (and the last release is from July) since they merged that feature into the desktop version of Firefox but IMO it does a really good job, you probably won’t get new languages unless someone forks it (or Mozilla brings the feature natively to Android too), but it’s pretty functional already.