According to the manual switching the built-in lucene search engine to some other language than english is easy. Just put for example:
%%prettify 
{{{
jspwiki.lucene.analyzer = org.apache.lucene.analysis.de.GermanAnalyzer
}}}
/%
into the jspwiki.properties.

But how to retrieve the GermanAnalyzer? I did not find pre-compiled .jar archives with the contributed Analyzers anywhere in the net.

So you've to head over to: [http://archive.apache.org/dist/lucene/java/] and download the appropriate Lucene source-code version. I think JSPWiki 2.8.2 uses Lucene 2.0.0. Alternatively you can use SVN and check out TAG or BRANCH 2_0_0 from: [http://svn.eu.apache.org/repos/asf/]

Just run the ANT-Task /contrib/analyzers/build.xml with the command "jar" and there will be "/lucene-analyzers-2.0.1-dev.jar" in your build path. Now you've to copy this JAR file to /WEB-INF/lib/ directory of your JSPWiki installation and you may choose the right Analyzer for your language.

I've attached my pre-compiled version of Lucene Analyzer to this post.