Languages available in the game depend on the region and edition.
You may check if a language pack for The Witcher 3 is available for you in the Xbox Store. If you cannot find it there and the language is not available by default, your edition of the game does not support it - please note that in order for a language pack to work it has to match the region and edition of your game.
If the game you purchased does not match your region, please contact the retailer to purchase a game for the matching country.