Translate interface

I’m translating some pieces of my WordPress site. I need to translate the interface of it (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.