A California court isn’t going to uphold EU travel rights. Try r/Legaladviceeurope

Suing under EC261/2006 in the US has not been successful https://www.govinfo.gov/content/pkg/USCOURTS-ca7-14-02414/pdf/USCOURTS-ca7-14-02414-0.pdf

You would need to sue in the Netherlands, or possibly Italy.

I’ve tried all this over the last several months, and sent receipts and my claim in November. They have not sent compensation and deny owing the €600 because they blame air traffic control, even though we were on the ground in Amsterdam for the entirety of the delay that caused me to miss my connection.

There is no need for a suit. It is all arranged here in Europe

First the airliner needs to take care of stranded passengers. If they are unable you can do it yourself and claim back the expenses. You need to keep the receipts ofc.

Second, depending on your situation you will get compensation 

Clearly explained in link below. And you can file the expense & compensation claim via de KLM website

https://europa.eu/youreurope/citizens/travel/passenger-rights/air/index_en.htm

I’m trying to figure out how to start the suit, specifically where. The issue I’m suing over is that I was delayed over 24 hours during my layover, which cost me about €500 plus I think entitles me to €600 under EU travel rights.

I need to file a lawsuit citing EU travel rights against KLM (Dutch airline). The flights took place last September and were booked through a travel company called EF Ultimate Break. I departed from Rome, had a layover in the Netherlands, and eventually landed in Los Angeles. I assume courts in Los Angeles would have jurisdiction but want to check before hiring an attorney in the wrong place. Will supply more info if necessary. Thanks!

Jurisdiction for Suing an International Airline