import re, sys

pLu = '[{}]'.format("".join([chr(i) for i in range(sys.maxunicode) if chr(i).isupper()]))
pLl = '[{}]'.format("".join([chr(i) for i in range(sys.maxunicode) if chr(i).islower()]))

text = "She has an excelllent command on the topicsOnly problem is clarity in EnglishHer confidence is very good in RUSSian and H2O"
print( re.sub(fr'({pLl})({pLu})', r'\1. \2', text) )
print( re.sub(fr'([a-z])([A-Z])', r'\1. \2', text) )