Make sure you are connected to the internet and fire up the Google Maps app.
Once in the app, let the map for your region load. You can zoom out for faster loading, but the finer details will not be loaded until you zoom and scroll.
After you are satisfied with the map being displayed on your screen, tap the search bar on the top. The search bar is used to find locations by name, but in this case we are going to use a hidden trick.
Type ok maps in the search bar and press enter. The search bar will now say Pre-loading map…0%. If you have internet connectivity, the percentage will go up and the app will save the map you have cached so far.
Once you see the confirmation message. It is safe to turn off the internet. You can verify whether the map has been saved by closing the app and running it again. Zoom into the map to see if all details are visible and scroll about to confirm.
You are not restricted from saving any map from any other region and can simply scroll to France for instance and let the map load before repeating the trick above. However, it is recommended that you cache only the region you need, so that the data is not overwritten.