The full title of the Byzantine emperor was :
"πιστός εν χριστώ τω Θεώ βασιλεύς και αυτοκράτωρ Ρωμαίων"
"pistos en christo to theo vasilefs ke aftokrator Rhomeon"
"faithful in Christ,to God king and emperor of the Romans"

PS:Many inscription on coins are still in latin even in the 10th century !