I have tried to find out how to solve this.
Wow, is this simple little page complicated. I don't know why there is so much going on on this page. I guess this has to do with bootstrap. I had and am still having a hard time to find my way around the html code and the css code for what is supposed to be a pretty simple page.
With adding display:none to #s2id_loginlang I managed to hide the drop down. But as the label "language" has no ID nor a unique class, I am not quite sure how to hide this via CSS. Maybe someone else can skip in, who is more up to date with coding or has experience with bootstrap.
Code:
#s2id_loginlang {
border: none;
padding: 0;
display: none;
}
Should be found in lime-admin-common.css