My source quotes this differently:
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.
Translation: In fact, another man, who was German, reported that Charles V himself used to say at times: if he had to speak to God, he spoke in Spanish, for the Spanish language conveys gravity and majesty; if with friends, in Italian, for the Italian dialect was familiar; if he needed to soften someone up, in French, for no language is gentler; if he needed to threaten or speak harshly to someone, in German, for the entire language is threatening, harsh, and vehement.
This “used to say” indicates to me that he might have said something like this, more or less, which was found funny and witty (counselors knew when to laugh when the king speaks, just like today), and he repeated it occasionally, with variations.
But others quote him differently:
“I speak Italian with the ambassadors; French with the women; German with the soldiers; English with the horses; and Spanish with God.”
I think this quote was made up a posteriori, as he seems not to have spoken English. And he spoke Spanish badly and only late in his life, almost nothing when he came to Spain to be chosen their king. Many criticized his lack of knowledge of the language, and many spin doctors (a term that did not exist then, but it was common practice) invented many an anecdote.
Fact is: he was a bad king and a bad person. The history of Spain and Europe could have turned better had he not been chosen king of Spain. Or maybe not, we’ll never know.