woocommerce-rest-api-docs/source/stylesheets/_lang-selector.scss

.lang-selector {
	a {
    border-top: 4px solid $lang-select-bg;
    padding: 10px 20px;

    &:active, &:focus, &:hover {
      border-top-color: #873EFF;
    }

    &.active {
      border-top-color: #873EFF;
    }
  }
}