how to enable a language in drupal

How do yo enable a specific language for H5P authors in drupal. i have set the whole Drupal site to italian e.g. and that works, although i still see a lot of english. But that is not a very elegant way to do it. Also, i would like to enable it only for H5P learners so they see navigation buttons in a specific language etc. in Italian (or any other language for which there is a translation available). How does this work exactly? We have installed the italian language files for Drupal already.