From http://semantic-mediawiki.org/wiki/Template:Language

This template is used for stating the language for some content page.

Usage:

{{LanguageSet|master page=Page title|language=en}}

The parameters have the following meaning:

• master page: the «main» page about that topic, usually the article with the English documentation for the most current version
• language: a standard language code

While a page is shown inside another page by Template:ShowArticle the showarticle variable is set and nothing is annotated, so the shown article's master page does not land on the showing page. see Property:Language code