<div class="outer">
  <div class="inner">
    some very very very very very very long text
  </div>
</div>

outer{
  position:absolute;
  width:100%;
  height:40%;
  background: rgba(0,0,0,.5);
}
.inner{
  position:absolute;
  bottom:10%;
  left:50%;
  max-width:100%;
  transform: translate(-50%, 0%);
  background: rgba(0,0,0,.5);
}