I agree it's difficult to be generic here and the case is less important than for iframes (imho). l10n_date.js could extend and redefine l10n's normal `translate` to do this. But this is probably not important to do this yet.

I wonder if we should be generic though, as we only have one single case in the Sms app. Maybe we should just update `.not-downloaded-message` elements in ThreadUI when we get the localized event ?

(I don't like attribute selectors ;) )