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http://meta.wikimedia.org/wiki/Help_Forum#Wiki-
Markup:_language_of_Wikimedia_article
suggests, there is nothing said in the mediawiki help pages about 
how to tag pages, sections, paragraphs, sentences, or single words 
of foreign languages. The short answer given on the page mentioned 
above could be good starting point.

As mentioned in the above URL, use <span lang="xx">your text</span>
where:
- 'your text' is a text in any language
- 'xx' is a code representing the language used.

If there is nothing said about it on mediawiki.org, feel free to
add it somewhere :)

Probably a duplicate of 10736 and/or bug 30422.