We lived in Strasbourg years ago, though my school experiences were in Montpellier.
I was really reluctant to put DS in the maternelle at age 3 as I hated the thought of such a structured environment all day from such a young age (plus I'm from NZ where they start school at 5 so it really did seem very young). Class sizes were about 28 in the local state school and 32 at the Catholic school down the road (with a teaching assistant in each case) - I did also look at the Steiner kindergarten (jardin d'enfants) which only had 12 per class, but I couldn't really deal with some of the more way-out aspects, like not having any books, plus it was a lot more expensive).
In the end though, I realised that literally all the other children his age would be at school, so he would have been starved of contact with his peers. I don't know that there would even have been much available in terms of childcare options for him. I guess for me a lot would depend on how many 3yo would be in your jardin d'enfants.
In the end we sent him to the Catholic school as we felt more comfortable there, but I had a very long talk with the principal about the level of religious instruction and was very open that we aren't Catholic. In fact that side of it was fine, there was really just a tiny amount at Easter and Christmas.
What really helped was that I only put him in for the mornings and then picked him up at lunchtime, gradually building up to 4 afternoons in GS. He did take a few weeks to get used to the maternelle, but he is a pretty sensitive soul and (now 8) has always found change difficult. After that he loved it and the teachers were really great, and it was more play-based than I had expected.
I guess the other aspect is not so much what will be best for development, as how hard it would be to slot into MS without having done PS - though my son had a friend who had been at the Steiner kindergarten (much less structured) and joined in GS - it went ok although he found school pretty boring in comparison and the teachers probably found him a bit too lively!
If you do go for the international school, you could ask how they handle bilingual children. My kids are now at the European School in Luxembourg and the older one has been doing French as his second language since he was 6 - kind of boring for him as he is bilingual already, but they do try to get him involved in helping the others.