There isn't a way within mediawiki. however if you use the site trick in google "term site:wikipedia.org" you should be able to do it
On Sunday, November 16, 2014, svetlana svetlana@fastmail.com.au wrote:
What URL can I use to search across all languages of Wikipedia (or a sister project)? Would like to add it as a search engine so that I don't have to constantly switch between 2-3 language different search engines in the list.
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