Luxembourg bank account - for what year does reporting start

When you complete your annual tax form you have to list any countries in which you have (potentially) interest bearing bank accounts, although I seem to recollect that you don't have to give any further details.

Ask the bank where you hold the account...

If you haven't reported that you have an account in Luxembourg when you complete your Belgian tax return for 2014 during the course of 2015, the Belgian fisc is going to be very suspicious when, during 2015, they learn direct from the Luxembourg's that you have an account there. I suspect that you may have some difficulty in persuading them that you only opened it during 2015 and therefore that it didn't have to be reported on your 2014 (and perhaps earlier) tax returns.