[trikadin]

<input id="need_to_check">
<input type="button" style="visibility: hidden" id="button" value="Mr. Button">
<script>
document.getElementById("need_to_check").onkeyup= function(){
 var st = document.getElementById("button").style;
 if (/[А-Яа-я]/.test(this.value))
  st.visibility= "visible"
 else 
  st.visibility= "hidden";
};
</script>