From February 30:
Why isn’t the current ruler of Spain known as King John Charles in English? We have King John I of Aragon and Kings Charles I to IV of united Spain, but the man who claimed the throne in 1975 was King Juan Carlos.
Read it all, as the great bloggers of this world say.
As a rule, Russian uses Old Church Slavonic names for monarchs; with no OCS version available, the standard Latin form is preferred. OCS is pretty faithful to the Greek source in transcribing names. John the Landless thus becomes Ioann Bezzemel’nyy, Jean le Bon, Ioann Dobryy, and Ivan Grozny (in pre-1917 official literature), Ioann Grozny. Louis NN-th is normally referred to as LyudOvik (from Ludovicus, apparently); Charles the Beheaded, as Karl I, and Elizabeth II, as Yelizaveta Vtoraya.
(More in comments to the entry above.)