Insurance for carless cyclist in California
I found one insurance that will be of interest for Germans:

The insurance broker OSD offers a liability insurance for German nationals that live temporarily abroad, for up to 5 years. The insurance company is called BDAE - Bund der Auslands-Erwerbstätigen, they work with Würzburger.
The liability explicitly covers bicycles "4. aus dem Besitz und dem Gebrauch von Fahrrädern;".

It has 5 million Eur coverage and is much cheaper than the renters insurance, about 100 Eur per year.

I haven't yet received a written confirmation from the U.S. based insurance that they cover bicycle liability, I'll post it when I get it. I am quite convinced that renters insurance would cover bike accidents, here is an older page with some good comments. 303 Cycling news

Update:

I went with a German international liability insurance from Allianz. It is valid for up to 5 years abroad (no time restrictions for the EU), and the clock is reset with every visit back. They confirmed this in writing.
It costs about 90 Eur per year. The max liability amount is 30 M Eur.

The U.S. renters insurance was State Farm, they confirmed on the phone that they would cover bike liability, but they never replied to me when I asked for a written confirmation.
It would have cost $270 for $2M coverage.
Many of the other insurance companies in the U.S. only offer relatively meager amounts of coverage (~ 100k), so I would recommend a closer look at State Farm for anyone interested (please post an update in that case), I am certain that they would cover bike liability, I just did not want to dig through the insurance policy when they can't be bothered to reply.