This is being worked on and is actually already available on Firefox for Android Nightly and Beta - check out @Andre_Natal's post in this thread for more info.
The Firefox-translations add-on is pretty good you can translate websites but also type text in text translation. But the only downside is that you don't have all the languages. Some are completed and some are in beta. It's more western languages then eastern languages. The most important for me is Japanese but is probably one of the difficulties. The difference between google translate and Firefox-translations is that the google version translation is maked by a bot but the firefox-translations is maked by humans. Everything by firefox-translations is translated by humans that's why it has not many languages because they need people who can speak, read, write that language. https://addons.mozilla.org/en-US/firefox/addon/firefox-translations/