Can't speak for Germany. Mexican customs is also well aware of what they are and won't hassle you for your personal use stuff, even though they're technically still considered contraband. As long as you're not trying to import a suitcase full of new, ready for sale Isticks or something, they're cool. They do advise all lithium batteries be transported in your carry on, and out of the device if possible, though I wasn't clear on exactly why (was explained to me in very fast-spoken Spanish). Something like "easier to control in an 'event' if they're in the cabin".
When I returned from the US recently, I removed from packaging and packed my newly purchased IPV 3, Istick, 4 18650 batteries (as safely as possible in little plastic cases which I recommend), and 2 ego C's in my carry-on. My two partially filled Kayfuns, EVODS, around 100 ml of bottled liquid, charger, empty boxes, and other assorted junk went into my checked baggage. No leakage whatsoever, except a very minute amount from one of the bottles, and no hassles from customs.