Navigate to your Library and select The Witcher 3.
If you own the game on GOG, adding the Japanese language pack is usually a straightforward process through their official client: thewitcher3wildhuntjapaneselanguagepackgog 58 top
If you are downloading the pack manually from the GOG website, make sure the file version matches your game version exactly to avoid "file mismatch" errors. The Impact on Immersion Navigate to your Library and select The Witcher 3