"도시의 불빛은 밝지?"
Translation:The city lights are bright, aren't they?
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.
This isn't very accurate. Without the question mark "도시의 불빛은 밝지" would translate to "The lights of the city are bright". With that logic, adding the question mark would just change it to "The lights of the city are bright?".
Adding "...aren't they" would, be "도시의 불빛은 밝지, 그렇지 않아?"
In this example "도시의 불빛이 밝-지", -지 functions more or less as an Eng. 'tag question', i.e. turning a statement into a negative question
The city lights are bright -
도시의 불빛은 밝-다.
• Negative question / tag question
Are the city lights not bright? - 도시의 불빛이 밝지 않아요?
The city lights are bright, aren't they? - 도시의 불빛이 밝-지?
These kinds of question are used when both interlocutors share the knowledge of the matter discussed, and the Speaker just wishes to get reassurance/confirmation from the Listener to the fact stated.
The question mark "?" is used to indicate the given example is a question. This is to avoid any ambiguity.
As you pointed out punctuation marks are mostly optional in Korean because the conjugated verb ending already define the nature of the sentence.
My guess is "The city lights are bright?" is viewed as a colloquial form of "Are the city lights bright?" where a statement is turned directly into a question.
The city lights are bright? = 도시의 불빛이 밝습니까?
The -지 ending has 2 tasks. First, it negates a statement; then it turns this negated statement into a question.
Statement: 도시의 불빛이 밝-다 = The city lights are bright
Step 1 (negate statement): 도시의 불빛이 밝-지 않다 = The city lights are not bright
Step 2 (question form of negated statement): 도시의 불빛이 밝-지 [않습니까] ? = Are the city lights not bright? <=> The city lights are bright, are they not?