There are two parts to this answer. First, localeCompare is a method on a string that takes another string as an argument and returns negative (before), zero, or positive (after).

    x = "ape";
    y = "cat";
    z = "dog";
    y.localeCompare(z);  // negative: "cat" before "dog"
    y.localeCompare(x);  // positive: "cat" after "ape"

It turns out that different languages (English, Swedish, French) sort strings differently, depending on the language. See Using Locales

Amazing!