Apple - 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.

Since iOS 13, you can now change each application's default language, and that is possible for Google Maps too, by going to Settings > Google Maps > Preferred Language (at the very bottom of that screen). You should be able to select one from the multiple languages you have installed on the iPhone (by default, of course, Google Maps will use the iPhone's 'standard' language).

If you don't see any other language there (besides the 'standard' one, i.e. the one the iPhone is displaying everywhere), then you can add more languages by going to Settings > General > Language & Region > Other Languages... and add there all the languages you wish; then go back to Settings > Google Maps > Preferred Language and the new language(s) should be available for selection!

Source: https://ios.gadgethacks.com/how-to/change-languages-per-app-basis-your-iphone-ios-13-0208770/ (last accessed 2019-11-22)