Interwiki / Wiki family: full text search
Do you intend to create *one* multilingual wiki and *many* unilingual wikis? The former is like http://meta.wikimedia.org (see the search page there for instance) and the latter is like the family of Wikipedias. In the first case, there is only one main page defined in the system message MediaWiki:Mainpage; in the second in the corresponding system message of each wiki. For the search, it is ready out-of-the-box in the first case, but not easily possible in the second case, you would need to deeply modify the code and set up a proper search engine to search accross all your wikis.
If you absolutely want "multilingual search", the first solution is recommanded and you can see multilingual extensions (Polyglot, Multilang, LanguageSelector). The second solution aims to have independant wikis, linked together but with a single language by wiki (and is adapted mainly for big wikis, since there are communities by language).
