Manual:$wgLegacyEncoding

Details
Set this to the character set, which you used before you updated to MediaWiki 1.5 (eg 'ISO-8859-1'). MediaWiki will then perform a temporary character set conversion when loading old revisions, which are not marked with "utf-8" flag (that are revisions created with MediaWiki 1.4 and earlier). You can use this when you convert a wiki to UTF-8 and you do not want to do a mass conversion of old text data. Note however, that old texts, that have been converted because $wgLegacyEncoding was set, are not updated in the database! The next time they are needed, they will have to be converted again.