@Sparkle If a town like Saint-Malo (pop. 45 - 50,000) can find people who speak different nationalities, I'm sure others could too and could set up a similar association. Yes, it's necessary have to have good French and preferably to be mobile. It's much easier now that restrictions on visitors to hospitals have been lifted.
Our bénévoles can say that they prefer not to take certain cases - some are more sensitive and would be nervous/uncomfortable in certain situations. Equally with, for example, Moslems, we try to have a man interpreting for a male patient and a woman for female patients. We are sensitive to other cultures and it's interesting what you can learn ! I once had to interpret for some English-speaking refugees who were having a "lesson" in contraception. I saw things I'd never seen before ...
Most of the English-speaking patients here are elderly, have lived in France for years but have never bothered to learn the language. The medical people are very relieved to have us ! It's also comforting for the patients to have someone of the same culture.