After a little more research, I'm a little confused. There is no Mediawiki:Subtitle, do you mean Mediawiki:Sitesubtitle? If so, that doesn't include the site title. I think you're talking about Media:Tagline, which can be set to whatever you like, you just have a manually input the site title in the appropriate language (using /lang if required). You can change the site title in other places with Mediawiki:Sitetitle.
As far as I can tell from looking at the code, all Mediawiki namespace pages can use the /lang form, there is no need to implement it for specific pages.