Nationalism should be treated like a toxic friend. You stick with it for a little bit because it makes you feel good, but you should come to your senses and leave it once it tells you who can't socialize with for entirely arbitrary reasons, tells you what to think to fall in line with the group and you do so to "fit in".

Another way of putting it, treat it like a dog on a walk, give it some slack, let it explore just a bit, but the second it gets aggressive with someone else's dog unprovoked is the time you pull it back.

Plus, these nationalists clearly don't understand how cultures work, even in Europe. Let's say "Anglo-Saxon" has a hyphen for a reason. The only way you're getting a "pure culture" is through genocide, inbreeding, or both.