use tabs for indentation instead of spaces to be like the rest of mediawiki?
I was going to say the same thing. Why wasn't that caught in code review?
Spaces were chosen because that is what we get with FormatJson::encode and there is no way to change it except by post-processing. I'm fine with both tabs and spaces.
Unless you change FormatJson::encode (which is used by many other things as well), you would need to apply this post-processing in multiple places, making it harder to alter these files from PHP code and you would take a small performance hit for the extra processing.
Also, I see we've lost the helpful comments that used to be in some of these files to visually divide things into sections.
That is true. JSON does not allow comments.
On the other hand, we can now stop updating messages.inc.
-Niklas