I've created a test site using Agent to handle English (default) Spanish and Italian. Works like a dream.
However, in Web Inspector every language version shows the page to be lang="en" despite using Agent's Lang Redirect stack to change the default lang to es or it (when either is the chosen page language).
I may have misunderstood how to set up this redirect. (I have a correct htaccess file in place.)
Redirect setting
Difficult to describe but I hope you get my drift!
https://safepay.calligram.design/?lang=es