Interesting. Took me a while to find the “read to me link” under the header on mobile (Android) - so ended up testing in chrome first. Here I for some reason got a very naturally sounding, Japanese accented English reading. By that I mean the English was atrocious - but sounded quite like a native Japanese speaker reading aloud in broken English. I suppose I have a Japanese voice set as first option in chrome, for some reason.
Firefox gave me a fairly good English robot voice - perfectly understandable, if not perfectly natural.
Yeah, my implementation is not perfect yet. It should choose the first voice which language starts with the HTML lang string (so with “en” on that post or “de” on the German part of the blog). The naturalness depends on the voice. In Edge on Windows there are voices available that use an online service and sound more natural. On my Linux installation it sounds horrible.