Translate interface

Hello:
I want to translate my wordpress blog, but not everything. I need to translate the interface of the site (menus, some strings, dates, widgets…), pages and taxonomies, but not posts.
I need the category “dogs” to be read as “dogs” for English, “cans” for Galician and “perros” for Spanish, but they all must be related to the same posts. It is important not to duplicate posts.
It should be as in Facebook or Twitter, where you can choose a language for the site, but the posts or tweets are always the same.
All best.