There's also downloadable CSV lists of the world's place names at https://geonames.org (and stably have been kept available and updated for many years by now) — if you already have a DB (likely in your case), script up some csv-to-dbtable dumping and thus you save on networked API call overheads just for querying the world's existing place names.
(It's regularly updated so best to keep the convert-and-dump script or logic around if you go that route.)
There's also downloadable CSV lists of the world's place names at https://geonames.org (and stably have been kept available and updated for many years by now) — if you already have a DB (likely in your case), script up some csv-to-dbtable dumping and thus you save on networked API call overheads just for querying the world's existing place names.
(It's regularly updated so best to keep the convert-and-dump script or logic around if you go that route.)