Languages available in the game depend on the region and publisher.
Make sure that the game disc, the language set in console and your PlayStation Network account's region/country match each other. You may check if a language pack for the Witcher 3 is available for you in PlayStation Store.
If you cannot find it there and the language is not available by default, your version of the game does not support it and it is not possible to enable it for your copy of The Witcher 3.
If the game you purchased does not match your region, please contact the retailer to purchase a game for the matching country.
If you wish us to check the region of your game disc, please provide CUSA code which is printed on the top of it.
Game purchased digitally matches the region of PlayStation Network account it was purchased on.