A wine bag is simply a clear plastic bladder with a spigot. There is nothing written on it and there are no labels (in case you reuse a bag that was originally sold with wine, this is all on the outer cardbox which you will discard). You can label the bags easily and very clearly with a sharpie. So the bags are not any more likely to be confused with beverage containers than any other unlabeled clear plastic container or unlabeled glass bottle would be.
The 10 liter size is easily available through the local German site of that big online auctioneer. You could always split up 20 liters on two bags. Actually, I would even recommend using four 5 liter bags for better handling.