The untranslated key icon is very useful, but only when you are responsible for the translation of every single language. I work in a project where I am responsible for only one language and the "warning" icon doesn´t tell me if my language is missing a translation or it is other language. So I end browsing every single key and this icon doesn´t do it´s job.
It would be nice if you could "hide" languages that you don´t want to translate or select only a set of languages to work in. These hidden or unselected languages shouldn´t be checked for missing values.