Google Translate fails to translate documents

For some reasons document translation does not work on https://translate.google.com/ when using Qt WebEngine. There is no error, but the result is not translated.

I was able to reproduce on a fairly recent build of the dev branch using the "simplebrowser" demo.

It works fine with other chromium browsers.

Steps to reproduce

1. Navigate to https://translate.google.com/?sl=auto&tl=fr&op=docs
2. Upload a PDF (you can use the default PDF from https://mozilla.github.io/pdf.js/web/viewer.html)
3. Click on the "Translate" button to start the translation
4. After the translation is done either download the translated PDF or open it. In both cases the PDF is still in its original language.