It looks like this affects all Ember apps. For example, even Ember’s own documentation app
It looks like Google is proxying the site, injecting some Javascript, and injecting a <base> tag. We might be able to detect Google’s hacks and workaround the 404 issue… but I very much doubt any complex JS app like Discourse will work well through Google’s proxy.
A workaround is to use the translate functionality built in browsers (either native, or via an extension). Those do not have the same proxying/injecting behavior.