Click the 'More' button (next to the Play button).
Once downloaded, you can go to the game's Language settings to set "Audio" to Japanese and "Subtitles" to your preferred language. Troubleshooting Common Issues thewitcher3wildhuntjapaneselanguagepackgog 58 top
If you own the game on GOG, adding the Japanese language pack is usually a straightforward process through their official client: Click the 'More' button (next to the Play button)