Why are the Robot names like R2D2 and C3PO in Star Wars using the alphabet/characters used in languages spoken on Earth, particularly Roman alphabets and decimal numbers?
The official language spoken in the galaxy is English (reasoning, see *), English uses the Latin alphabet and Arabic numbers. Thus the galaxy uses the Latin alphabet and Arabic numbers.
(*) There are two ways to see this. First of all, people in the senate speak English (in the original version of the film). Apart from the fact that it's the language you hear (in the original version, not the translated, obviously), you can tell that some members of the senate (e.g. Jar-Jar) have a bit of a hard time using it correctly and make some grammar mistakes.
That whole argument still holds, if you say that what you hear is a seamless translation from some StarWarsGalacticLanguage (SWGL) to English (for the sake of the film) because then, also the designations of the droids would have to be translated from some weird SWGL symbols to Latin+Arabic.
The universally accepted answer for any question such as this is the translation required between the alien language and English uses a close approximation in sound and/or meaning to provide a contextual mapping between the languages.
Or in other words, the above is a load of rubbish because it's not real.