As for the -locale part of the extension, they are used by few. So some of the larger ones have been separated as to not punish the many with a larger download, and they are very easy to locate and load, no real need for adding them to dep files really.
I think a reasonable criteria is that if a locale pack is around 5mb compressed, and is over half the size of the main extension, then it makes sense to keep it separate. Otherwise it can just be included.