In Big Sur Apple was using a mix of flags and other icons. The flags seem to be for countries that use the Latin alphabet.

Playing around with the Keyboard Preferences, I've found:

Different Chinese inputs use different sinograms: the one for Pinyin uses the symbol for "Pin"

all Hebrew inputs use the Hebrew letter aleph

all Korean inputs use the same Korean symbol

all Thai inputs use the same Thai symbol

all Arabic inputs use the same Arabic symbol

Greek uses a lambda for one input and an epsilon for the other Russian curiously uses a Russian flag and not a Cyrillic letter