I think you can enter GPS data, but it won't give location, saying "Looking for location…"
However, I just did this with a Library on an external drive-- I entered an address; I entered GPS coordinates-- in both cases it found the location and showed me a map!
OK-- rather than relying on my memory, I went back to stuff I did a couple of months ago:
I have a small test Library from my internal drive that I copied to an external SSD. I opened that in Photos, and I entered a location in one of the scanned images that didn't have one. It worked fine. I used the Image>Location procedure to copy the location from that picture to another, and it worked.
However, in another picture I entered Eiffel Tower into the location field of a picture; it correctly placed the Pin in Paris, but "Looking up location info…" appears in the field.
When I copy that Library back to my internal drive, the Eiffel Tower shows up after a short wait.
It looks like pictures that had previously successfully used "Lookup" to name a location have kept the name. But new name lookups don't occur on the external drive.
So, apparently it's not simple! It looks like recording the location data before import works for you.
As to the Eastern Europe vs Utah, thing-- I know that the GPS format can be weird-- like what + or - means, or having the E or W in the right place. Was it just East longitude instead of West? We've also seen lookup fail in some places that are not well surveyed, or at least well traveled by Apple people.
Until the external drive thing gets fixed, here's an idea from what I do: I have a separate Library for my Nikon pictures. I do all the editing and curating in the Nikon Library, and then I transfer the finished pictures to the Library that is connected to iCloud. You might want to do something similar-- have a small Library on your internal drive that gets all the new pictures, curate them there, and then transfer. Even if the external Library can't get the Lookup right, we know that the information is there, and it transfers between Libraries. With multiple Libraries, most of us use the trusted 3rd party app PowerPhotos ($30) to make it easy to move albums back and forth.
Just thinking…