How to change turn-by-turn navigation voice language in Google Maps for iPhone?
Unfortunately, Google Maps doesn't have Language controls built into the app. It cues off of the settings in Settings.app > General > International.

The only way to have Google Maps speak a different language is to change the actual settings in Settings.app > General > International > Language. 

Of course, this doesn't take long to change, and it may be desirable to change it before using turn-by-turn for perhaps a long trip.

It would be a good idea to request that feature from Google by opening the Google Maps app and tapping the person icon in the upper right corner. Then tap "Send Feedback" and compose a request to Google.

Minor Update

You can now set voice search language in the app. There is a mic icon on the right in the search box that you can tap to basically dictate the location. While that doesn't apply to your question, perhaps that would be a first, mildly helpful step.