When a language pack is not available on your system - the first transcription will trigger the download of the >500MB language pack. Depending on your internet connection this might take a while to complete the installation.
You can also manually trigger the installation via the Creative Cloud Desktop App: - In the "All Apps" View - click on the "..." icon next to Premiere Pro
- Choose "Addons"
- This will show the list of already installed language packs
- click the "add" button next to the language pack / this will trigger the download process and show you the progress made so far