I dunno, when DW & I got engaged, we picked the date FIRST, and then booked the rest off that. If there is a difficulty finding venues, caterers, etc, it seems they're only compounding the issue by failing to nail down a date.
We were opposite. We knew the church we wanted to be married in and the venue we wanted for the reception and a general month. We found out what the availability was at those locations and then picked the date both were open.
Nine months out, especially in Germany, is pretty early to be planning a wedding. A long engagement is rare over there. I think it’s fine to ask that they let you know the date as soon as possible, but I wouldn’t be looking for deals until they give you the date (it just leads to disappointment at missing great deals)