.header__menu-item span{
text-decoration: unset !important;
position: relative;
  font-size: 18px;
}
.header__menu-item span:after{
content: '';
height: 2px;
width: 0;
background-color: currentColor;
position: absolute;
bottom: -5px;
left: 0;
-webkit-transition: width .25s;
transition: width .25s;
}

.header__menu-item:hover span:after{
width: 100%;
}