I want to buy an iPhone, and would like to ask whether the iPhone 6 Plus has either Persian or Farsi language support.
Apple eventually added the Persian keyboard and language to the list of the iPhone keyboards and preferred languages back in 2017. Now all iPhones with iOS 11+ can add Persian keyboard and or language in their iPhone
Settings > General > Keyboards or
.. > Language & Region.
Prior to iOS 11, Persian users could install the Arabic keyboard and long-press ک - ج - ب - ز - ي keys to input the Persian letters گ - چ - پ - ژ - ی.