I am happy to report that there is a solution to this! In working with support last night we were able to perform certain queries and updates to certain tables contained in the the portal gwdb, all via the postgres command line interface.
The updates will remove and then add languages, and upon completion of the workflow, the English language pack is fully available, thus enabling (for us) the Instant Apps template and config pages, as well as restoring Map Viewer Symbol symbol categories. I cannot speak to a re-gain of the Esri Basemaps content as we did not lose that part first reported here
https://community.esri.com/t5/arcgis-enterprise-portal-questions/basemaps-unavailable-after-upgradin...
by @ShelbyZelonisRoberson
I am not able to relay what we did exactly, but anyone affected by this should be able to work with their support analyst now to affect this repair.
Support cannot yet say whether the fix will be included as a patch for 11.2 now or as part of the 11.3 release (or both) - they just don't know yet.
@ThomasColson
FYI the Instant App issue for the Sidebar app does still exist, where the 'Sidebar' has become a "Footer' and the Filter tool is non-functional, and some other css issues, so looking forward to 11.3 for this fix, @KellyHutchins
