I installed backdrop (www.pdfvg.net/bd) but I don't understand how install italian language?

and wich version of Drupla language I have to use


I think your best approach will be to use the various Drupal 7 packages for Italian available here https://localize.drupal.org/translate/languages/it. Note that some modules are now in Backdrop core but you can still import the separate module files found on that site. For example, Views, Token, Pathauto, Date, Link, ..

Then import them into your site.

Some strings have probably changed a bit so you still might have some manual translations to do.

The Backdrop localize site is still a work in progress so sadly you won't be able to use it yet.