Does the woman speak French?
*** Spoiler ***
Does the kid's mom know French? If she doesn't, how does she communicate with her Swiss boyfriend? If she does, why does she need to get his letters translated?
At that time (the early 1970's) lots of young people used to come and volunteer in the Kibbutz (as seen in the movie- there were volunteers from Finland).
Many of the volunteers come from Central Europe and french-speaking countries in praticular (Switzerland, Belgium and France).
The mother AND the child understand and might speak basic french, just as the rest from the Kibbutz are capable of. That's also explains why the letters were off to translation, as formal french cannot be handled by them.