A colleague of mine just copied this regex from above in our codebase so I want to make an update here that nowadays browsers do widely support unicode.
var regexp = /^\p{Letter}+$/u;
var val1 = "Normal";
var val2 = "Połącz";
var val3 = "Illegal space char";
console.log(regexp.test(val1), regexp.test(val2), regexp.test(val3));
// ^--- true true false