Whenever you open a tab, you'll be asked to translate a word. 104 languages are supported.
With the 'New Tab: Learn English' extension, you'll be prompted by a simple word question(s) whenever you open a new tab. Every question you answer will be added to your dictionary so you can keep track of how many words you have learned using this extension. Currently, 104 languages are supported (this number is related to Google Translate API).