That's a video by Bob Black. Here's Bob's solution, in text form. I've had the same partial success as others: the voices work great in P2A, but not in the random pool. Non-English voices do speak English, with an appropriate accent, adds realism for flights outside the USA.
In brief: Install a new voice. Copy the voice's registry key,
from HKLM\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens\
to HKLM\SOFTWARE\Microsoft\Speech\Voices\Tokens\.
Restart P2A.
Bob's procedure:
Windows 10: Start>Settings>Time & Language>Language: Add a language.
Type the name of a language. (If you type something other than English, the controller will still speak English, but with an accent.)
Choose a language that has a speech balloon icon (text-to-speech), and install it. (Bob's video says you need the microphone icon (speech recognition). That doesn't seem to be the case; it's the speech balloon you need.)
Get the voice's nationality and name from the registry: you should know how to use the registry editor if you're going to do this. If you don't know yet, it's worth learning. The risks are over-stated, but do be careful!
HKLM\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens\
Names like, e.g., MSTTS_V110_frCA_ClaudeM for French Canadian.
Export the MSTTS… key to, e.g., frCAClaude.reg, in some handy folder.
Open that folder in File Explorer. Right-click and edit the .reg file in Notepad, Notepad++, or your favorite code editor. (Double-clicking will by default re-import it, don't do that yet.)
In the text editor, change the key name to remove the "_OneCore" string. Should be two places, change'em both. Save the file.
Doubleclick the file (or Open With: Regedit), and approve merging it into the registry.
Restart Pilot2ATC (since this takes a while, do several at a time, once you get the first one working.)
P2A: Config, Voices tab, assign the new voices to controllers. Haven't figured out how to make them work as members of the random pool.