deleteEqualMessages: Refactor to support all langcode subpages

deleteEqualMessages: Refactor to support all langcode subpages

Previously it ran for either the base pages (comparing against
content language) or 1 lang-code subpage.

And the latter was actually broken as it forgot to add the
subpage back on, thus resulting in incorrect deletions
(deleting MediaWiki:Foo instead of MediaWiki:Foo/nl).

Also fixed a bug where it used the subpage lang-code to create
a Language object and call ucfirst(). Page titles should only be
capitalized by content language.

Bug: 48050
Bug: 43917
Change-Id: I2680413c276365a44c935a6f6fdd740daa86341e