After you have launched the game at least once so that Ubisoft Connect gets installed/updated, do the following:
Go to your Steam Library, find the game and right-click it, choose "Manage" and then click "Browse local files".

First of all delete the stupid Ubicrap Launcher installer file which you won't need anymore named "UbisoftConnectInstaller.exe" (the ".exe" part might be hidden, stupid Windows defaults...). That's 244 MiB already.

Next open the subfolder "Data" and then "Paks". There delete all the Audio*.pak files EXCEPT for "Audio-common.pak" and your desired language - in my case "Audio-en_US.pak".
So I deleted "Audio-de.pak" and so on.
That's the meat of the wasted space - for me it was 1,83 GiB.

Unfortunately updates will restore those files but I wouldn't expect too many any more, unless that damn launcher installer file gets updated, like today... -_-
Well, at least this gave me an opportunity to write this guide! xD

I hope this guide is easy to follow and helped you!

If you have constructive feedback please don't hesitate to leave a comment, cheers!