MediaWiki 1.19.0 points out that it will perform better if pecl-intl is installed. It is not part of the Portage tree:

http://pecl.php.net/package/intl

Specifically, the installer says this:

Warning: The intl PECL extension is not available to handle Unicode normalization, falling back to slow pure-PHP implementation.
If you run a high-traffic site, you should read a little on Unicode normalization.