The whole issue of fuel containers is a bit of a legal nightmare. Within the EU fuel containers require markings and specific design features and there is a blurry line between "Personal emergency use" and mass transportation. There are also local laws. Basically, if you have one green plastic 2-litre of the sort most petrol stations sell you should be fine, anything else like bladders or metal jerry cans could be an issue.
Chances are you'll never need to talk to anyone about it though. The German plod won't be bothered to debate my 20-litre NATO spec can just for the fun of it, it's complex for them too and they know I'll claim that what's legal in the UK is legal in Germany as far as vehicle mounted stuff goes. If they'd pulled be for speeding they might add it the charge sheet. It's only if I close the Brenner pass tunnel for a few years by catching fire they might make a real song and dance about it. If the can or bladder is leaking or insecure of course it all becomes painfully clear in the legal mind.
I'd go with what works, keep it out of sight or generally safe looking and empty when not really required.