\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage{comment}
\includecomment{mention}

%% set-language-environment UTF-8

\begin{document}

Yes:
\begin{mention}
Ð
\end{mention}
.

No:
\begin{comment}
Ñ
\end{comment}
.
\end{document}
