AFAIK there are almost no prerequisites ETH is insisting on other than a Master degree in a related field of your prospective research.
However, you have to understand that you're really not applying for the school, but for a position. Unless there is some kind of special PhD program you are applying to you will only get admitted as a PhD student when you already have a corresponding job at some institute. The way to go is to apply directly at the different research groups for a position, if you want to do your PhD at ETH. In the end it will depend on what the professor offering the position wants to see of a candidate. Usually I would say they don't care for language certificates, but I wouldn't count on it.
Best,
Michael