I have few queries after installing silverstripe locally :

1) If I want to add the functioanlity of multi-language then how could I go ahead with it. Does silverstripe has such pulgins/add-ons etc, so that I could use it, if not then how could I do it.

2) If I want to modify some header contents, like I want to add slider or anything, then how could I do it, becuase at admin side, they just given simplly 2-3 options just to change the names and like that.

3) How can add new themes, can you please tell the procedure.

Any help would be highly appreciated.

1. Have a look at the Translatable module at https://github.com/silverstripe/silverstripe-translatable

2. If you are referring to the "Settings" section of the CMS ("admin" section), you can extend and add extra fields/data available, see this area of the SilverStripe documentation http://doc.silverstripe.org/framework/en/reference/siteconfig

3. Themes are simply folders in the "themes" which contain html, js, css and include the SilverStripe template tags to insert the data. See this section about the template engine http://doc.silverstripe.org/framework/en/reference/templates

Hope those are useful links for you.