There are some extensions which are not developed in English at all. But that are rare. At least in Extension Warehouse and the forums I use. It might be that they confine themselves to other sites local to those languages.
Extensions localized in multiple languages is a hit and miss. I think we (SkethcUp) can do more to encourage localisation.
Recently I started using CrowdIn for translating my own extensions - and that made it much easier for people to offer translations. And they even offer their services for free for open source projects. If extension developers starting using this offer it could be easier to attract people to help with translations.