RSS feed page for localized SO sites uses favicon from enSO. E.g. ruSO uses favicons with blue color:
But if you open RSS feed from these sites you'll see icons used in enSO:
for feed from main ruSO used favicon as for main enSO:
for feed from meta ruSO used favicon as for meta enSO:
Tested on Firefox and Chrome browsers for Windows.
I think favicon for feeds on localized sites should be same as for sites itself.