Attempt to use load_plugin_textdomain() before loading translations from WP_LANG_DIR

This will ensure compatibility with future support for automatic translation
downloads on wordpress.org/extend, while continuing to allow translations to be
stored in wp-content/languages/.

Fixes #5030

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7204 cdf35c40-ae34-48e0-9cc9-0c9da1808c22