I think there is something else that needs to be done to get all of the translation strings working!
I say that because, following the successful upgrade of our test system we proceeded to upgrade the production system.
This time we cleared the browser cache as recommended above, and as expected *most* of the error messages disappeared.
But not all of them.
Trying to find out why, I searched the codebase. Using the example of the error message "string code asset_map_tree1_name not found", this what I found:
The string "asset_map_tree1_name" is not in the file "js_strings.en.js" (and I checked our working test system, which is no longer showing the error.)
The translation "Tree One" for the string "asset_map_tree1_name" is only found in the file " lang_strings.xml"
Anyway, while that searching was going on, we continued to work with the production server, adding pages, making things live, changing others to "under construction" .
and after about 4 hours, all of a sudden, the remaining error messages disappeared.
So all in all we're pleased that it is all working well.
But we have no idea, what we did or what happened to make it work.
Thank you everyone for your help
(BTW: the string for "asset_map_tree1_name" is still not in the copy of "js_strings.en.js" from the browser cache)