I'd be kind of silly for "standard time" to be based out of a planet that doesn't even exist in their galaxy, wouldn't it?

Ultimately, "basic" would be whatever language YOU speak. The idea is that there is a basic language used throughout the galaxy, obviously it isn't intended to be English, since that is an earth language and therefore from an entirely different galaxy, and developed much, much later (since SW is a long time ago), but as with most sci-fi, it is the convenience of storytelling that makes this language be English. If you were Russian and watching a Russian dub of the films, or translation of the novels, then basic would be Russian.

In other words, even though the story teller doesn't intend for the language to actually be your language, it would be unreasonable for him to invent a fake language, and make you read subtitles for the duration of the film.