If you turn it on once, I don't think you can do it in Amethyst. At least, I couldn't manage it unless I deleted Amethyst, which then loses the settings, or by clearing the app data.

You can't turn it off fully. You can do it for languages one by one. (e.g. if a Spanish post is translated, you can press on Spanish and you'll get option for "Never translate Spanish").

Keep doing it and with time, you won't see translations.

There was auto-translation in Amethyst? Where can I find it?

If you install the fdroid version there is no auto-translate

Install the fdroid build instead of the gplay build, it doesn't feature translations.

Is that a new feature?