Use 7-Zip (WinRAR may fail on multi-part archives).
Pro tip: If you get a CRC error on part 58, your download is corrupted. The "top" indicates this part holds the table of contents for the speech files. Re-download that specific part from the source. thewitcher3wildhuntjapaneselanguagepackgog 58 top
Solution: The "top" 58 file is incomplete. Ensure you have all files from 58_top.001 to 58_top.060. Use 7-Zip with "Combine and Extract." Use 7-Zip (WinRAR may fail on multi-part archives)
A: If you own the base game on GOG, the language pack is free. However, version 58 top may require manual downloading from community archives. Pro tip: If you get a CRC error
Launch the game. Go to the main menu. If you hear Kenjiro Tsuda say "Gwynbleidd" in the intro cinematic, the pack is installed correctly. If the game crashes on load, the "58 top" speech bundle is mismatched with your executable version. Download the 1.58 top hotfix (often posted alongside the voice pack).