It would really be nice if WordPress had a built in function that would allow each user to select his or her preferred language for the admin interface.
I'm using the WP Native Dashboard plugin for this, and it does the job well for a vanilla WordPress install. But any other localized plugins I add end up using the site language setting rather than my personal preference.
I don't know if an improvement to WP Native Dashboard would rectify this issue, but it seems to me this is something that WordPress really should have built in. We don't really need all of the functionality offered by WP Native Dashboard, just a per-user preference setting for the admin interface language.