mw.widgets.CategoryCapsuleItemWidget: Handle non-English foreign wikis

mw.widgets.CategoryCapsuleItemWidget: Handle non-English foreign wikis

To be precise: when the language of the foreign wiki is different than
the language of the local wiki, and it's not English, things go bad
when we're checking for page existence (to display blue/red link).

Just assume that any namespace-like prefix is the 'Category' namespace.
This is a horrible hack, but it's better than throwing exceptions.

This doesn't fix the fact that suggestions in mw.widgets.CategorySelector
are missing, but that isn't causing exceptions, so it can wait.

Change-Id: I3ed11a9d7a1d87c0f43c0a988fe4a8db5b8a907c