Not really experienced any localism as such and I've surfed most of the Atlanctic coast of Europe (other than Ireland - next autumn hopefully) as well as my North Sea home.
But generally I avoid the over hyped breaks and anywhere with a rep for localism -unless its mega sweet and then I'll go in early morning when it's less busy. But generally speaking there's just a good break 5 miles down the coast with almost no-one on it and friendly locals.
I'm never agressive, paddle out and wait out a set or 2 and let everyone see you're not a git. I learn the local language - say hello, smile and be cool. But i do also seek out the emptiest breaks at the emptiest times - I love surfing alone.
Here's my classic example of what not to do to avoid localism. I was in southern Portugal in late autumn last year, urban beach break slight rep nothing too heavy,solid 4 foot, steep fast take offs and clean faces, nice day for good surfers. I'm out with maybe 10 locals on three peaks, me and two locals sharing one peak. we weren't exactly best buddies but sharing waves with no BS. British guy paddles out with his girlfriend, doesn't acknowledge anyone. He's pretty good she's a learner and can't paddle into the waves. So he pushes her into a few , ok not ideal but everyone's gotta earn, but then he pushes her into a wave that one of the locals is already on. Literally drops his girlfriend in on local guy's head, and no apology, nada. Ok a mistake i can live with it. Then he does the same trick again everone gets FO'd and vibe ruined. Travelling surfers get bad rep at the spot, next learners may get some hassle in the water. The worst thing was the brit surfer was actually pretty good and should have effing well known better. Sorry rant over i'll chill now.
Actually the only time I've ever had agression in the the water it's been from other travelling surfers dropping in, madly paddling for every wave and generally acting like gits.