If you change this in the downloadsplugin, you must uncheck the downloadplugin in staticpages, safe and recheck the plugin, safe. In this case staticpages will be load the new link from the database. I know, it is not the best way, but i working on it.
I changed the name from 'downloadmanager' to 'downloads'in the language file for the plugin (was this correct?). I then followed your instructions but it still shows in the static pages menu as 'downloadmanager'