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 Nintendo eShop - please note that in order for a language pack to work it has to match the region and edition of your game.
If you want us to check the region of your game card, please provide code which can be found on the back of the game box, above the barcode or on the game card itself (e.g. HAC-P-AURVL).
Digital downloads are connected to the eShop region where they were purchased.