Andrea

Press your windows key and type "speech".
Move down to "speech settings", and press enter.
Then tab over to the "Manage voices" group, and you should be on the "add voices" button.
Press enter on that, and you will be able to enter and choose your language, and then install the voice pack(s) that you want.
At minimum, you'll want to install/keep the English (US) voice pack.
Once you select and install the voice pack(s) you want, close everything and restart NVDA.

Luke