BuddyPress.org

The Notifications %s string looks like it is available:
https://plugins.svn.wordpress.org/buddypress/tags/2.6.1.1/buddypress.pot

If you’re using a custom translation from a previous version, you need to merge your language files together. Here’s a quick tutorial I found on Google:
http://www.marketpressthemes.com/blog/how-to-merge-two-po-files-using-poedit/

You might also benefit from a translation that is already available here:
https://translate.wordpress.org/projects/wp-plugins/buddypress