How to use Tamil language in my website? I have installed the language selection page module. I have Tamil, English language to be chosen by users. 

If user chooses English means site will appear in English Language. If user chooses Tamil language means site will appear in Tamil language. 

So, how can I install Tamil language in Drupal 7.34?
Use Internationalization module to make your site multi language. It gives you translate interface for every language you select in configuration. 

After enabling the module you have to add Tamil as language under admin/config/regional/language/add.