Thank you Rosemary and Maira for your assistance on this! I was actually wondering about this French vs Luxembourgish issue. I read somewhere that some teachers would be particularly strict with children not speaking Luxembourgish at all and that would make the whole transition harder. That's why we're also considering a crèche that speaks French in the first 18 months and later switches to Luxembourgish. However, from Rosemary's words that doesn't appear to be a real problem.