Switch the :lang(nl) and ::before parts, ::before should always come last.
I fixed it. It was the right code bits but in a different order. To change the language of the empy list text:
.MyReservationsList>ul>li.mx-listview-empty{
&:lang(nl)::before {
content:"Er zijn nog geen reserveringen voor de komende periode.";
}
&:lang(en)::before {
content:"There are no reservations yet for the upcoming period.";
}
}
.MyReservationsList>ul>li.mx-listview-empty label{
display: none;
}