Charles V, Holy Roman Emperor (Spanish: Carlos I, Dutch: Karel V, German: Karl V.) (February 241500 – September 211558) was effectively (the first) King of Spain from 1516 to 1556 (in principle, he was from 1516 king of Aragon and from 1516 guardian of his insane mother, queen of Castile who died 1555, and the co-king of Castile 1516–55, full king 1555–56), and Holy Roman Emperor from 1519 to 1556.
I speak Spanish to God, Italian to Women, French to Men, and German to my Horse.
Charles V may have said something in this general format, but not with this specific wording. Variants have been quoted for centuries, and the earliest known citation, itself a secondary source dating from 40 years after his death, gives two versions that both differ from the modern one. Girolamo Fabrizi d'Acquapendente's 1601 De Locutione gives:
Unde solebat, ut audio, Carolus V Imperator dicere, Germanorum linguam esse militarem: Hispanorum amatoriam: Italorum oratoriam: Gallorum nobilem ("When Emperor Charles V used to say, as I hear, that the language of the Germans was military; that of the Spaniards pertained to love; that of the Italians was oratorical; that of the French was noble").
Alius vero, qui Germanus erat, retulit, eundem Carolum Quintum dicere aliquando solitum esse; Si loqui cum Deo oporteret, se Hispanice locuturum, quod lingua Hispanorum gravitatem maiestatemque prae se ferat; si cum amicis, Italice, quod Italorum dialectus familiaris sit; si cui blandiendum esset, Gallice, quod illorum lingua nihil blandius; si cui minandum aut asperius loquendum, Germanice, quod tota eorum lingua minax, aspera sit ac vehemens (Indeed another, who was German, related that the same Charles V sometimes used to say: if it was necessary to talk with God, that he would talk in Spanish, which language suggests itself for the graveness and majesty of the Spaniards; if with friends, in Italian, for the dialect of the Italians was one of familiarity; if to caress someone, in French, for no language is tenderer than theirs; if to threaten someone or to speak harshly to them, in German, for their entire language is threatening, rough and vehement").