I have an app that supports only german & english languages and I need to force a map (or even more - geocoder) to be in one of those languages.
While using RES
Google Maps iOS SDK uses the system region/language settings but doesn't expose any way to manually force any particular language. Geocoding responses (using the SDK classes) and the map labels both reflect these device settings, thankfully.
(Current as of 1.9.2, released February 2015.)