From what I have seen viewing PianoPedia, I think it's also the Chronological order pieces were made, so effectively earliest to latest. Just to be confusing I noticed some composers have 2 systems, Mozart with the KV and KKV or something O_o Sometimes one is a revision, or someone else has compiled the works into an order, perhaps not in a chronological fashion.
Also... Scriabin I think it was... or Scarletti, one of the "S" names has 2 systems too, god knows why though :/
Concurred on the Latin ^_^ I would be confident saying that was language.
Might as well compile the friggin things and find all the different translations:
Bartok - SZ (For some reason I have SZ.49 Allegro Barbaro)
Mozart - KV
Mozart - KKV (Or something similar to above)
Bach - BWV
Bruckner - WAB
Chopin - B (I have some B Numbers among some Piano Solo Variations)
Chopin - CT (Similar to the above, this is used with the Hexaméron Variations)
Rachmaninoff - TN (I think these were numbers for the Transcriptions only?)
No doubt I missed a load of others, considering the amount of composers there are :/ Apologies if I have anything wrong!