I know that Indonesian does not have a system of tenses, but could "pergi" also mean something subjunctive or conditional, as in "If you were to go, I would go"?
ok, i try to google subjunctive for the meaning, but i thinks it's beyond me :)
but, to translate your sentence:
If you were to go, i would go (too)
Jika kamu yang pergi, saya juga pergi
We don't use the subjunctive very much in English any more. It is the form of "were" above, or for instance, in "if he were rich." A present subjunctive is often "be," as in "I insist that she be here." Some languages have a full set of subjunctive forms, which is a huge pain for an English-speaker and must be even harder for an Indonesian-speaker.
I think "leave" should also be accepted. "If you leave, I leave too". Or maybe even "If you leave, I will leave too".
leave has a different word, 'berangkat' it would change the meaning of the statement, as it stands the subject could be talking about going to a shop or a concert but to change it to leave would change the meaning. I hope that makes some sense
I understand what you're saying in that it would make the English meaning a little less vague and give it more specificity that the Indonesian sentence does not have. But I have come across other questions where both "leave" and "go" are accepted, since they can be synonymous in different contexts, so there should be more consistency with this on Duolingo.
In fact, I even just tried "Jika Anda pergi, saya pun pergi." in Google Translate out of curiosity, and Google translates it to "If you leave, I leave." If you reverse it, Google translates that English to "Jika kamu pergi, aku pergi".