I think many people including me want to have the Japanese translation option for websites. I need to use now google translate for translating Japanese websites. And I don't like it to use every time google translate for that.
We have held back the release of these models on Android for now, since they are a bit larger, slower, and use more memory than the models we have released previously.
We have a plan and are working to improve the performance characteristics of using these models on Android.
In the meantime, you are welcome to try these models in the Firefox Beta and Firefox Nightly applications on Android.