Map settings

To my knowledge, the language or voice that you set under iPhone Language (Settings -> General -> Language & Region) is the voice you have on Maps. You can't change that.

If you want, submit your request to Apple directly, using his link: Apple – Product Feedback

Regards

Giulio

What App are you using for navigation exactly?

Apple Maps, Google, Waze?

Each has its own way of changing the voice.

Apple Maps uses the voice assigned to Siri for instance. Other Apps would have other voice settings.