I went through this precise exercise twelve years ago, and have had to do it a few other times since then, all with successful outcomes. I am not an attorney. But I've done a crap-ton of research.

First, the document that acts as the primary framework for international travel between and amongst the airlines and the countries in which they provide conveyance of carriage, or transit, is called the Montreal Convention. Most all governments are signatories to, regulate international travel.

This document sets up a framework for the handling of interline baggage, transfer of passengers from one airline to another, and various other nuances of maintaining operational cohesiveness. It also says one very important thing: "litigation may be initiated in any court of any country in which the airline operates." So, you have standing. Standing is the ability to bring a lawsuit against a party.

Second, look at the local rules that govern transportation in the country that you are travelling to and/or from. If you are en-route through a transit point, such as an EU country, that affords you the EU regulations, which have been discussed in greater particularity elsewhere on this thread, and do not need to be re-discussed.

If you are travelling entirely within the United States, however, the governing body of regulations, is dictated by DOT250.5 ("i.e. you are not entitled to compensation if the airline is able to place you on another flight or flights that are planned to reach your next stopover or final destination within one hour of the planned arrival time of your original flight.").

Go to the Superior Court website of (whatever county you are in) and look for "how to file a small claims action". Then file it, along with a letter to the court to ask the court to "serve the defendant(s) on your behalf", along with the filing fees. Don't forget to ask for the cost of your filing fees in the complaint.

Here's a couple of examples.

Lawsuit against United for an involuntary denied boarding situation. I received the full amount requested, absent the amount requested for infant passenger (who court says was not inconvenienced at all). LOL. :-)
http://customer.files49.com/private/sc100_united.pdf

Lawsuit against American for failing to provide conveyance of carriage. Received full amount requested:
http://customer.files49.com/private/sc100_american.pdf

Lawsuit against Alaska Airlines for failing to provide value of cancelled tickets toward another itinerary, and the follow-up for sharing information with a third party provider, in this case Expedia, for purposes of issuance of tickets. Recovered full amount requested:
http://customer.files49.com/private/sc100_alaska1.pdf
http://customer.files49.com/private/sc100_alaska2.pdf (* I dropped this claim at the bequest of Alaska Airlines after they agreed to refund me the ticket value, in cash.)

Lawsuit filed against Orbitz (Expedia) and Lufthansa, for failure to refund the monies owed on a ticket that was cancelled because of COV, and in particular, inability to deliver upon contract for carriage to/from Morocco. This one is pending and will be heard in May.

http://customer.files49.com/private/sc100_orbitz.pdf

I hope that you will be able to recover some of the monies owed to you. Basically, a contract for carriage is a contract for carriage, and it does not matter how, or why, the contract can not be fulfilled, especially when that reason is beyond your control.

I understand some people will flame here here for using the judiciary in such a casual manner to enforce the rights that I have, as a passenger, per either DOT or EU rules. However, it is *very important* that consumers assert themselves, and in doing so, they are bringing the level of expectation up to a higher level of accountability for all of us.

Finally, I am curious whether Expedia and Lufthansa will show up to court, considering that Lufthansa has abandoned the San Francisco market. They may wish to send someone who is not affiliated with their company (such as outside counsel), which I will ask the court to strike their testimony and/or bar their appearance for being a non-company employee. Basically, small claims litigants are not allowed to be attorneys who are acting on the behalf of another, non related entity. They have to be employees.