Google text-text translation API

I intended to build a language translation system (Mobile/Web Platform).And i found out that Google translation API could be use/ apply in the system. But I'm not sure if the Google Translation API is free or i have to pay for it. and what will be language we need to use while using the google translation API. Overall my concern is

1)programming language. 2)platform(web/ mobile). 3)Google API Is free ?