I am building a multilingual site with 3 languages. Lets say English, Italian and German. English is the default language. Is it possible to leave the admin area untranslatable, even I am currently seeing the actual site in different language other than the default one, just for the administrator of the site, using a module or custom code?
You could try the Administration Language module. There's only a dev version for the Drupal 7.x branch out currently, but it seems to work well enough (used it in a project myself).