Charles V doubtfully said it like this, though it would have a been a clever hat-tip to his domains — Spanish piety, Germanic martial culture, French the aristocratic common tongue. This blogger chronicles the 400-year evolution of the saying.
Italian seems originally to have been held in oratory esteem rather than a feminine one. “German to my horse” seems less than respectful. The earliest sources don’t mention a horse, and German is singled out for being relatively foul. For example, “If to threaten someone or to speak harshly to them, [I speak] in German, for their entire language is threatening, rough and vehement.”
I thought of the saying as I was rereading one of my favorite historical novels, Captain from Castile, in which Charles V appears as a character. He doesn’t speak or think the saying anywhere in the novel, but I can see where the “Spanish to God” part comes from. The scenes involving the Spanish Inquisition are powerful, and the general feel of 16th-century Catholic piety is chilling. The Spaniards may not have been as reprehensible as the Aztecs (Captain from Castile is refreshingly politically-incorrect, and true to history), but the worst elements of Spanish Catholicism at this time were certainly as bad as Aztec sacrifice, and it warms my heart when the Inquisitor gets his just deserts — roasted alive on the pyre of Xiuhtecuhtli (god of fire), a fitting payback to all those he burned at the stake (if the Aztecs only knew) back home.