I want to visit Vatican city and want to get a reduced entrance ticket. Is getting an ISIC card the only way, or maybe a university student identity document translated in English will be enough for getting a reduced ticket?
For the Vatican Museums and the Sistine Chapel, which are on a combined ticket, I think you'll find what you want here: http://mv.vatican.va/3_EN/pages/z-Info/MV_Info_Biglietto.html
Students up to 25 years of age, on presentation of a valid International Student Card, a student identity document or other documentation stating their registration to a university or institution of higher education, are granted the reduced price ticket
This reduces the price to €8.00 from €16.00 - price and entry info here: http://mv.vatican.va/3_EN/pages/z-Info/MV_Info_Orari.html
While it sounds like they'd accept any official documentation, given the long waits often involved in Vatican City ticketing, it's worth considering having the easily recognisable ISIC to speed the process up. But obviously don't buy the ISIC unless you're planning to visit many museums and other such places with student discounts since it costs more than the saving here. If you have a university photo ID card that's obviously a student card, most (not all) places will usually accept it without requiring ISIC.