— Charles V, Holy Roman Emperor Holy Roman Emperor 1500 - 1558
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"). <!-- sincere thanks to user Ehrenkater at Wikipedia Reference Desk for the translations-->