localize elements in database

I want to localize my website with .mo files. Now, while it's pretty easy to scan existing php files and add the relative translation, it's not clear to me how to localize the elements in the db. I don't mean posts but for example, i've some tables i use for compiling dropdown menus...what should i do? Insert elements this way: __( 'Test', 'test_domain' ) directly in the DB? If so, how can i import them in the .mo/.po files?

Then last question is: what is the function to switch the default language? I mean, the classic row of flags on which an user can click to switch language?