Do we want some kind of policy about multilingual namespaces? We could easily have a few hundred of them, all filling up the search footer, for one thing. For another, we might want to set a style guideline for how to identify a page as being in a particular language... which seems more directly what is desired than a translation of (say) all 16 standard namespaces into 50 languages.
I would prefer to see something like (8 namespaces) x (Discussion flag) x (Language identifier). At 50 languages, this makes for 800 'namespaces' but only 60 different options to select from.
How this shows up in URLs is up for debate, but an unambiguous option might be (Language identifier):[Discussion flag:][Namespace:](Pagename)
So the current main page would be En:Main_Page; a meta page might be En:Meta:Namespaces, a discussion of a german help page might be De:Diskussion:Hilfe:Handbuch ...
For the sake of aesthetics, one could rewrite such URLs in the current format, with a pretty and unique prefix for each combination of (Lang id, discussion flag, namespace). Then the above could be written as
Page:Main Page Meta-en:Namespaces Hilfe Diskussion:Handbuch
Importantly, in the database there would not be 800 separate namespaces; and when searching, one would choose from among the 8-or-16 core namespaces, and from among 50 languages; rather than from their product set.