Add in another language to existing website

Hi, i have an existing custom website built with Joomla! 2.5.11. We have allocated the selection of language button but the button isn't function yet and now we need someone who can help us activate the language button to switch to another language and in between. and also display all the pages in its own language; english = english , chinese = chinese. beside that, we will also need you guys to link the account to facebook & youtube (button also allocated, just need to activate the function)