\documentclass[12pt]{article}

\usepackage{fontspec}
\usepackage{polyglossia}

\usepackage{makeidx}
\makeindex

\setdefaultlanguage{slovak}
\setmainfont{Noto Serif}
\setsansfont{Noto Serif}

\pagestyle{empty}
\begin{document}
\thispagestyle{empty}

%L\index{L}.
%Ĺ\index{Ĺ}.
%Ľ\index{Ľ}.
l\index{l}.
ĺ\index{ĺ}.
ľ\index{ľ}.
%O\index{O}.
%Ó\index{Ó}.
%Ô\index{Ô}.
o\index{o}.
ó\index{ó}.
ô\index{ô}.

\printindex

\end{document}


\index{}.