2016-11-29: Support was added for Oppia. (Other news …)
I have updated the message.
It is interesting: see Category:%(newcat)s.
So? That's just what happens in MediaWiki...
The tooltip is in Dutch for me.
It's in Finnish for me. But I doubt that's what Тест8 meant.
May be, add : before Category -> :Category ?
That's what the revised message has, indeed.