LaTeX: How to have "This page intentionally left blank"

latex.this-page-intentionally-left-blank.tex

\documentclass[twoside]{scrreprt}


\makeatletter
    \def\cleardoublepage{\clearpage%
        \if@twoside
            \ifodd\c@page\else
                \vspace*{\fill}
                \hfill
                \begin{center}
                This page intentionally left blank.
                \end{center}
                \vspace{\fill}
                \thispagestyle{empty}
                \newpage
                \if@twocolumn\hbox{}\newpage\fi
            \fi
        \fi
    }
\makeatother

\usepackage[english]{babel}

\usepackage{lipsum}


\begin{document}

\lipsum[1-4]

\cleardoublepage

\lipsum[6-12]

\end{document}

In think you mean "This page is intentionally left blank." with the "is".

No, indeed,` it is grammatically correct to say "this page intentionally left blank". See for example https://english.stackexchange.com/questions/116053/this-page-intentionally-left-blank and also https://www.quora.com/What-is-the-grammar-behind-This-page-intentionally-left-blank-Why-is-is-omitted