From the end:

Amir E. Aharoni, 26/11/2013 10:01:

[The reason I say that the current situation "probably makes the
Installer get less attention than it deserves" is that I am not aware of
any easy way to compare translation statistics between different message
groups. Can anybody think of anything other than going to
Special:MessageGroupStats and manually comparing the numbers?]

If you think we should keep an eye on it, it should be added to https://translatewiki.net/wiki/Translating:Group_statistics

> Hallo,
>
> The MediaWiki Installer is in a somewhat unique position with regards to
> localization: Its code and messages are a part of the core repository,
> but it has a separate i18n file. The messages do not appear in the core
> group in TWN, and it is listed as an extension.
>
> It is not a part of the in the "Extensions used by Wikimedia"
> meta-group. That is probably correct technically, because these strings
> are unlikely to be seen by end users of Wikimedia projects (the only
> scenario I can think of is installing MediaWiki on labs instances).
> However, this probably makes the Installer get less attention than it
> deserves and this hurts other users of MediaWiki (see below about
> "probably").
>
> I thought about adding it to core, but it has around 300 messages, which
> is a bit heavy. Then I thought to list it as a separate group so that it
> would appear in the TUX groups selector when people would search for
> "mediawiki". Any other thoughts?

The "extensions used by Wikimedia" group already has some super-huge extensions which almost no wiki uses, e.g. EducationProgram. I don't see any reason not to add it there, where it won't affect the stats required by Wikimedia LangCom's language policy or anything else.