"도시의 불빛은 밝지?"
Translation:The city lights are bright, aren't they?
What makes this a question in the negative? Why isn't "Are the city lights bright" correct?
The 지 ending I think. You are staying a statement and then by adding the 지 ending you're asking for an agreement. Therefore "isn't it" or "aren't they" is added in English.
The final particle 지 is the reason why the translations adds "aren't they". The final particle 지 is added when you are asking a question and expecting some degree of agreement.
"Are the city lights bright" would be 도시의 불빛은 밝아? The difference being the final particle 아.
The differences are very minute (but there is a difference).
Please note, this is all informal. To make these sentences formal you would add a 요 at the end.