Oh, it's absolutely no problem, everyone has the same problem as there is no photopass in front of the castle or other land marks ;-) Usually near the castle you can find CMs nearby either for information or for cleaning.
In French it would be: "Pourriez-vous prendre notre photo, s'il vous plaît?" But asking in English, or just pointing at your camera will be understood.
The French are really attached to their language and a few years ago you could come across many who didn't understand English, but nowadays I have noticed an improvement in the level of English with CMs and guests.