My site default language is Hebrew (RTL), the problem is that it's all the site, and I want my sites admin-interface to be in English (American). how could I re-translate it to English?
There is a module for that: Administration Language.
This module lets the administrator see all administration pages in his/her preferred language.
You can use this to display the frontend of the site in one language and still keep most of the backend in English (or another language of your choice).
You can use the standard Languages page to choose the language of the admin pages.