Ok, that does put a different slant on things. In this country people have been borrowing money (mortgages) over a term that brings them right up to retirement. In these circumstances the last type of loan anybody wants is one where the term can increase beyond their earning years!
However it seems as though your term is going to start at 25 years from what you've told us. So assuming you're in your early to mid thirties or younger, you can probably cope with a stretch in your term. You should ask for an illustration of the effect a 3.5% rate increase would have on the rate.
One thing is for sure however, interest rates will go up, and with all the new money being printed over the past few years, they could rise substantially. I know Germany is dead set against inflation in the Union, but if and when it happens, it may be beyond their control.
Perhaps it's a mindset that comes with the type of set up your used to in a country, but I don't like the sound of it. Too many variables in todays climate. I know that in the states, you can have a fixed rate for the entire term, but then again, if you can't pay, you just give the keys back. Can't do that here, don't know about Luxembourg. Its not fo me though!
Patrick