Manual:$wgLegacyEncoding

Details
Set this to eg 'ISO-8859-1' to perform a temporary character set conversion when loading text of revisions, which is not marked with the "utf-8" flag. You can use this when converting a wiki to UTF-8 without 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.

NOTE! This DOES NOT touch any fields other than old_text and ar_text (and also does not change their content). Titles, comments, user names, etc still must be converted en masse in the database before continuing as a UTF-8 wiki.