Manual:$wgLegacyEncoding

Details
Set this to eg 'ISO-8859-1' to perform character set conversion when loading old revisions not marked with "utf-8" flag. Use this when converting wiki to UTF-8 without the burdensome mass conversion of old text data.

NOTE! This DOES NOT touch any fields other than old_text. Titles, comments, user names, etc still must be converted en masse in the database before continuing as a UTF-8 wiki.