I see the Github issue has been closed and deemed “not worth fixing”. I think this is more widespread than just the batch edit though. As reported by others, seemingly random languages for vernacular names appear in the emails as well. Even on the taxon pages this issue occurs. I see the French “Bleu Commun d’Europe” as the page title for that taxon, for example.
I checked my account settings and see my localization of common names was set to “North America (Continent)”. I probably picked that once upon a time as it is at the top of the list and I equated that with US English. I guess I am getting the Québecois I deserve!
So it seems like choosing large regions, sometimes even including oceans, can cause this. Switching to the United States as a country solved this particular taxon page for me, but maybe I will start seeing more Spanish and Tagalog from now on? I think the common names should be based on the interface language AND the region, rather than just the region.