Changing "language" settings used by Pandoc for generated output [Pandoc template variables]

You can pass the lang option to the template using -V which passes it further to the babel package.

pandoc -V lang=nl somefile.markdown

This is translated to the following LaTeX code which should result in dutch chapter headings.

\usepackage[dutch]{babel}

It's quite simple. You have to pass the following parameter to the pandoc call.

-V lang=de

You'll have to use language parameters according to BCP 47 (like en-GB, fr, de...)

The exact behavior is described here: http://pandoc.org/README.html#language-variables