why there is no space between the colon and the word 'Logging': Because Help: is the namespace name.
Ah. Silly me. I have amended the qqq page.
So, this is a help page, with many different names including 'Logging in' in English. I am not sure that I understand how this works when the site language is different to the interface language. I have tried to experiment here on translatewiki.net but I can't find the link.
I don't think this is worth spending too much time with. It's a very stupid idea of MediaWiki, that it links documentation where it does not exist (on the local wiki). We should work on Bug 43591 – Link translatable help pages on mediawiki.org from the relevant special pages, which is also the approach Translate uses for its own documentation.
I for one would prefer an "if the local page does not exist, then transparently deliver the central page from www.mediawiki.org" approach, mainly resembling the way, pictures are handled and included from Commons if needed.
Lloffiwr, Purodha: Steven said they're going to remove the link from MediaWiki core and to put it only in Wikimedia projects messages. Can you help making mw:Help:Logging in up to date and tagging it for translation? mw:Help:Editing pages needs help too, because it's going to be linked from MediaWiki:Edithelppage/en (it also has old translations to migrate).