Registration in Brussels - Ixelles

Hi all,
We are an argentinean couple and we will move to Brussels in September, and we need advice on how to register! We will do so based on a work permit and family reunion.
In some websites I read that we had to do it within the first 8 days after arrival, but other sites inform that we should do it within the first 3 months. By the way, we have a temporary lease contract for 3 months, is it necesary to have a permanent lease contract to register?
Finally, is it necesary to speak French in order to register at the Town Hall?

Thanks all for your help in advance!!!!
Cristina and Nicolas