这应该有帮助。 但是，您需要JQuery来实现这一点。

null

$('select[name=select1]').on('change', function() {
  $('select[name=select2]').prop("disabled", true);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="custom-select" style="width:200px;">
  <select name="select1">
    <option selected disabled value="0">Select Language:</option>
    <option value="1">HTML</option>
    <option value="2">RUBY</option>
  </select>
</div>
<div class="custom-select" style="width:200px;">
  <select name="select2">
    <option value="0">Select Language:</option>
    <option value="1">HTML</option>
    <option value="2">RUBY</option>
  </select>
</div>
<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>