I was looking because the one I was using was not updated to FF57.
I have seen his addon several times. And I did not realize it was the one I needed.
I think because the example images are not very enlightening. They are in Japanese.

So you need more pictures to explain the operation:
1 - Text selected in German, or Spanish, or French, with the button appearing.
2 - Box with the text translated in Japanese.
3 - Box with the text translated in English.
4 - Options with Japanese text.
5 - Options with English text.
6 - Toolbar box with Japanese text.
7 - Toolbar box with English text.

Add in Options the option "Traslate in New Tab".
A Near Tab. Not a Tab in the End.

Do not use the expression "Target language". I was confused.
Use the expression "Translate To". It's more clear.
The second language only works in the toolbar panel. It took me a while to realize.

Is the best.