Problem in the audio engine.

Hi everyone,
I'm experiencing a strange issue that's even hard to describe. I'm a Persian speaker, and since Apple's built-in Persian voice called Dariush can't pronounce English words properly, I installed a third-party voice engine from the App Store called Speak NG, which you might be familiar with. I've currently set it as my active voice engine because it reads both Persian and English very well.
In VoiceOver settings under Speech, I’ve set the Primary Voice to Speak NG - Max with the language set to Persian. Since I want to be able to switch between different voices depending on the language I’m reading, I also added English under “Additional Languages” and selected Siri as the voice for English.
Here's the issue:
When I use the rotor (Control + Option + Command + Shift + Left/Right Arrows) to switch between voices, everything works fine until I move from the primary Persian voice to English (Siri). At that point, VoiceOver becomes completely silent and doesn’t say anything. I have to rotate back to Persian (Speak NG) to get it working again.
One more thing: I’ve set the macOS system language to Persian. Interestingly, if I remove Persian from system languages, Speak NG stops working entirely—even though it’s still installed. I have to ask a sighted person to re-add Persian so the voice starts working again.
I’d really appreciate any help or suggestions to fix this issue.
Thanks in advance!