I just checked the ${JFORUM_HOME}/WEB-INF/config/SystemGlobals.properties, and it was set for Chinese environment:
lucene.analyzer.default = org.apache.lucene.analysis.standard.StandardAnalyzer
lucene.analyzer.chinese = org.apache.lucene.analysis.cjk.CJKAnalyzer
lucene.analyzer = ${lucene.analyzer.chinese}
You can check if this fix the problem:
1.Stop JForum
2.Change the last line as follows:
lucene.analyzer = ${lucene.analyzer.default}
3.Remove all the files under ${JFORUM_HOME}/WEB-INF/jforumLuceneIndex
4.Start JForum
5.Login as admin and go to Admin Control Panel
6.Regenerate all the index from scratch