"That if he were to speak to his Horse, it should be in High Dutch."

High Dutch is a dialect of German spoken in northern Germany and the Netherlands.

Emperor Charles V (1500 - 1558) was Holy Roman Emperor, King of Spain, and ruler over parts of Italy, the Netherlands and much of North and South America, as well as various islands. 

In discussing the chief languages of his dominions, he supposedly said that he would "speak Spanish to God, Italian to women, French to men, and German to my horse." This was an expression of his contempt for German, which he did not speak well.