Not working in any browser but one

Hi, thanks for a great plugin.

But weirdly translation only works in my Brave browser, and not in any other I have tried (Chrome, Safari, Firefox). When clicking on a flag nothing happens.

A friend tried in his Brave browser, and for him it didn’t work either.

I tried deactivating all other plugins but that doesn’t make any difference. Unfortunately I can’t switch themes as I have the plugin hard-coded in my handmade theme.

Hope you can help.

The page I need help with: [log in to see the link]