This extension creates a context menu item in the browser, when you click on a menu item, the previously selected text is sent to Google Translate. You can also setup default language in the option page.
First you should change the default language settings by accessing to about:addons > To Google Translate option page.
Select any text in some web page and then right click with your mouse
You can test the extension in Firefox with web-ext. web-ext can be installed with the Node Package Manager.
sudo npm install --global web-ext
Download the code from Github
git clone https://github.com/itseco/to-google-translate.git
cd to-google-translate
Run in the root folder extension the command
web-ext run