If you have access to the official store, this is the most reliable way to ensure all localized assets (like lip-sync and audio files) work correctly:

Highlight on your HOME menu and press the [+] button. Select Software Information > Nintendo eShop . Scroll to Downloadable Content and select View All Items .

: Use applications like Goldleaf or DBI to install the additional language NSP just as you would an update or the main game file.