Hello,
when article title contains non-ascii characters, they apear in URL as urlencoded (i.e. %C5%99 for 'r with caron'), which is not so easy to read.
AsciiTranslitExtension allows you to use Ascii aproximation (or so called transliteration) in URL, and nonASCII in titles (at the same time).
More: http://www.mediawiki.org/wiki/Ascii_Translit_Extension
vitas @;;