We have the exact same problem. We set the following in Info.plist :
<key>NSPhotoLibraryUsageDescription</key>
<string>The application accesses your pictures to allow you importing them in the current intervention</string>
And we set the translated one in different languages in InfoPlist.strings, for example:
"NSPhotoLibraryUsageDescription" = "L'applicazione accede alle tue immagini per consentirti di importarle nell'intervento in corso.";
The translated message is shown the first time where you have to choose between letting the app see some of the picture or all of them. If you pick the restricted option, next time the app is used and you open the gallery you have the UNTRANSLATED message in the alert. If you set <string>NSPhotoLibraryUsageDescription</string> in the Info.plist that is all your user is going to see.
This is definitely an iOS bug. It translates OK the first time so there is no reason it wouldn't work as well later.