It is not a function of web browser, I tried Mozilla Firefox 52.3.0 and Google Chrome 60.0.3112.90.
I downoaded again several other firmware versions from the website you suggested but none of them helped.
Finally, I downloded from the same link the Language files. Oddly enough, there is no English among these languages, but installing the Spanish language, with which I can handle, worked perfectly!
So, my conclusion is that this specific device's default language is Chinese, firmware update does not change language, and a language pack installation is the answer.
Now, if I could find an English language pack, that would be nice.