MediaWiki:Config-charset-help/en

Warning: If you use backwards-compatible UTF-8 on MySQL 4.1+, and subsequently back up the database with mysqldump, it may destroy all non-ASCII characters, irreversibly corrupting your backups!

In binary mode, MediaWiki stores UTF-8 text to the database in binary fields. This is more efficient than MySQL's UTF-8 mode, and allows you to use the full range of Unicode characters. In UTF-8 mode, MySQL will know what character set your data is in, and can present and convert it appropriately, but it will not let you store characters above the Basic Multilingual Plane.