I have never changed the games language but this is what I can find: If it is from the store you need to download the language pack from there and install it. If it's from steam you need to right click on KSP in your games list, select properties, select the tab at the top 'language', change to Italiano and then verify the games files: https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335