how can we unpublish item with its subitem for one particular language in sitecore?

Approach 1: In the Publish ribbon, click the Change button and UNCHECK Publishable on the Item tab. Now delete the parent item with sub-items and the item will be removed from the published DB. Next go back to the item and CHECK Publishable, then publish the item only in the language you want.

Approach 2: Use the database selector in the Sitecore shell (bottom right corner) and select your publishing target DB (e.g. "web"). GO into that tree, find the item in the language and delete that version. After that you'll need to go to your public site's cache page and clear that cache manually: http://host/sitecore/admin/cache.aspx

Approach 3: Delete the item in the master DB, publish the parent, restore it form the recycle bin, publish it again live but not in the language you don't want.