OK after much research, it appears that Google has crippled it's own translate app in 10. The "Tap to translate" button does not appear when copying text, but has been replaced by a "floating" icon which is present on the home screen all the time, and is not easy to use.. This CAN be turned on and off, but the settings are very complex and confusing.
I live in Mexico and this capability was very important to me, for translating texts and emails from Spanish, and vice-versa. Now my Android phone basically is the same as an iPhone in this respect.
I wish I had never allowed the update to Android 10.....