Sorry, I can't help with an IPhone.
But there must be something similar to what there is on my Android phone
Under settings
Download maps
and near the end an entry called
Voice prompts (TTS, preferred)
If you tap on this you see a (long) list of possible languages.
On the right of an installed language there are three vertical dots.
If you tap on these there is an option to remove this language package.
After that you can (re-) install it.
I don't know if there is a development plugin on an IPhone to test voices.
Hopefully someone reading mailing list owns an IPhone and can help with this
Helmut