For in-page translation of entire pages (without opening a separate tab): you’ll probably find that Google’s extension sometimes, or often, succeeds where alternative methods fail.
Alternatives
An extension from NewTranx Information Technology. For Firefox:
Unfortunately, the product in Chrome Web Store fails to install with both Google Chrome and Brave Browser. There’s an alternative version (probably inferior) at http://www.newtranx.com/Home/Index/index/id/8.html?l=en-us but this, too, can not be installed.