Of course "AD" is Latin, and the AD2015 order is probably appropriate for Latin.

I wonder if $20 comes from the appropriate order for Spanish, now that I know the $ symbol comes from Spanish!

My guess is that it comes from the order typically used for ledgers, which is Description followed by Quantity. So you might have a ship's manifest with entries like:

  | sacks of barley          |    130 |
  | kegs of wine             |     12 |
  | pesos de ocha            |  2,800 |

So the abbreviation Ps starts out on the left-hand-side too.

Nope. In Latin languages the number comes before the period (2015 DC). That was a hard detail of English to learn...

But in about any language the currency symbol comes before the number.

> Of course "AD" is Latin, and the AD2015 order is probably appropriate for Latin.

It'd be just as appropriate if Anno Domini was translated into English.

Funny: when movies in English are dubbed to Spanish, the Latin expression is expanded and translated also, placed before the number: "era el año del Señor 1212 cuando...". It sounds good as if it were an archaism (the normal way of speaking is "1212 después de cristo" or 1212 DC).