they look fine to me. "Still" has a slightly more intense emphasis - "Are youquote[still] here?" suggests that you've asked the question before, or the person has been here an unusually long time; (I *still* hadn't read the book; dad *still* hadn't met mom, the *plane* still hadn't left). "Yet" doesn't carry that implication.
Given the past tense, "yet" is seldom vital to the sense, but it's very common in conversational speech. There may be a Spanish equivalent, I don't know.