Translate Tweets By Google | Script Greasemonkey/Tampermonkey
Before you can install any of these scripts, you must first install a user script manager for your browser; GreaseMonkey for firefox or Tampermonkey for Chrome. There are also user script managers for other browsers. See the Greasy Fork's page on user script installations for more details.
Install the Userscript in TwitterGoogleTranslate.js
change the "ENTER-YOUR-API-KEY-HERE" for your api key (Google Translate)
change the "en" for your language
var direction = 'ltr'; // ltr|rtl
var textalign = 'left'; // left|right|center
var apiKey = "ENTER-YOUR-API-KEY-HERE"; // YOUR-API-KEY-HERE
var langSource = ""; // Detect language automatically
var langTarget = "en"; // The language Google will translate the text in.