I've just experienced something similar, which brought me to this thread. According to my preflight check, I need to install en-PG. While it's true that English is one of the official languages of Papua New Guinea, it is effectively the same as Australian English. Although my computer is physically in Australia currently, when I dug into System Preferences I found that my region (not preferred language) was set to PNG. So it seems that Publisher interpreted the region setting to mean it should be using a PNG-specific spelling dictionary. When I checked each text frame in the Character panel (as advised above), it revealed that the language was set to a generic English rather than English (Australia). As I changed the language setting for each frame, the corresponding error message disappeared from the preflight checklist...except for text in tables, which continued to flag the message even after the language setting was changed.