Using coins to unlock carts/trolleys in Europe VERY COMMON, even in UK (where ALDI is as well).
I still have a plastic coin in my wallet in case I come across the need to unlock a cart at a supermarket (Aldi, Spar, Sutterluty, etc in Austria).
I have shopped at ALDI both in UK and in Austria. This store is BUDGET style of store. Do not expect huge choice, brands, luxurious presentation!