If I use a german character (äÄöÖüÜß) in a wiki search, it's not possible to find the article.
I'm not sure, but it seems to be a general (MySQL/DBD/DBI) problem.
For test, just make a wiki-article with a title "Ä".
I tested this on the actual versions (stable and beta) and the wre with 7.6.33.