Is an ISIC card enough to get served alcohol in these countries?

I'm going to go with no, unless they are putting birthdates on them now. But, on the bright side, I know that in Germany the drinking age for beer is 16, and for hard liquor it's 18. If you just go into a bar or restaurant and order, you're not going to get carded anyway, EVER (I lived there for three years and never did). You sometimes need an ID to get into a club, but that's rare and a driver's license or school ID (with birthdate) will do that for you. I'm pretty sure it's the same or similar in the other countries you mentioned. They're a lot more accepting as far as that stuff goes. It's just part of the culture.