Add documentation for setMapLanguage()

[m0nac0]

Closes #519

[felix-ht]

LGTM

[hardgit]

It has no effect at all!

MapboxMap(
accessToken: "**********************",
onMapCreated: _onMapCreated,
styleString: "mapbox://styles/mapbox/satellite-streets-v11", //卫星地图
initialCameraPosition:
const CameraPosition(target: LatLng(34.72468, 113.6401), zoom: 3.5),
onStyleLoadedCallback: () async {
addCircle(mapController);
mapController.setMapLanguage("name_zh");
},
)；

[adplant]

It has no effect at all!

MapboxMap( accessToken: "**********************", onMapCreated: _onMapCreated, styleString: "mapbox://styles/mapbox/satellite-streets-v11", //卫星地图 initialCameraPosition: const CameraPosition(target: LatLng(34.72468, 113.6401), zoom: 3.5), onStyleLoadedCallback: () async { addCircle(mapController); mapController.setMapLanguage("name_zh"); }, )；

Agreed. I've tried with mapController.setMapLanguage("name_ar"); and see no change at all.