No, not all swiss speak french. In fact, a majority speak only swiss german. Here is a map to help you undersand what languages are spoken and where they are spoken in Switzerland:

While this is technically true, it is not 100% that Mercy would usually speak any languages other than Swiss German and English. Most people tend to stick to the language that they know unless they travel a lot, and as she speaks German preferentially, it looks like she's Swiss German.
What is less excusable is her accent, which sounds High German just like Reinhardt's and which doesn't make a lot of sense unless she hasn't lived in Switzerland for a very long time. A real Swiss accent would be a lot sharper than hers.