iOS Safari SpeechSynthesisUtterance can not set language
I have to little rep to post this in the comments so i will post it as an answer. I see that you are running iphone 6s, but i tried it on my iphone X and on my Mac with both english and italian in different browsers.
There is a possibility that the problem persists within your phone or os because it seems to work perfect on my devices, have you tried any other devices than your iphone 6s?
This issue seems to be purely for the beta. The same HTML works fine on iOS 11.4.1 for the iPhone 6, 7 and 8 Plus. By changing the language to Spanish, however, it works perfectly.
<html lang='es' class=''>
Of course, this would mean your whole site is considered Spanish, but it seems to be one or the other if you insist on using the beta.