Wouldn't these branches be permanent? They might be fine for translations, but I doubt upstream would accept one for a bug fix.
As for bookmarks, if I understand this correctly, I have to convince upstream to use them.
With git I can create a branch in my repository and upstream can pull my changes in whatever branch it wishes too. I don't need to convince anyone to do anything else, besides accepting my patches.
I wouldn't use 2 separate repositories for that, just one.
We've got multiple named branches at work (I also use bookmarks with development branches fairly heavily, but that's a slightly different topic).