High Galactic alphabet

The Latin alphabet was a form of writing in the Galaxy.

It may even have been a fairly commonly used alphabet, as Obi-Wan Kenobi asked Anakin Skywalker whether he had a "plan B" when they became trapped by ray shields during their attempt to rescue Chancellor Palpatine.

Also suggesting the widespread use of the Latin alphabet is the naming of the so-called Rebel Alliance and New Republic -wing starfighters, such as: A-wings, B-wings, E-wings, X-wings, Y-wings, and Z-wings, all of which bore some resemblance to their Latin alphabet letter namesake.

Droid names often use Latin letter names (verbally), as R2-D2 and C-3PO are pronounced ARE-TWO DEE-TWO and SEE-THREEPIO, respectively. Another is Emtrey, from M-3PO.

Behind the scenes
The Latin alphabet is a form of writing in the real world and is the most common alphabet of Western nations, usually containing about twenty-six letters. It is somewhat unsurprising that this alphabet, especially its American English variant, is in the Star Wars universe, as the Star Wars movies and most Expanded Universe materials are of American origin.

However, it must be noted that unlike Aurebesh, Latin alphabet is never mentioned or taken into consideration. Roman labels in A New Hope were edited into Aurebesh in the DVD release, indicating that Lucas had some reasons to believe that Latin doesn't belong in the Star Wars saga.

It's possible that all occurences therefore should be ignored from our mind as artistic license, and be considered as 'translated' Aurebesh. On the other hand, Roman letters can be seen alongside Aurebesh, or are very meaningful, as in the case of the -wing ships.

Appearances

 * Star Wars Episode III: Revenge of the Sith
 * Star Wars Episode IV: A New Hope
 * Star Wars Episode V: The Empire Strikes Back
 * Star Wars Episode VI: Return of the Jedi
 * Numerous places in Star Wars Expanded Universe media.