>most of those people probably do not understand the Koran's language (which is a more than thousand year old version of Arabic).

My understanding, as a non-Muslim who doesn't speak Arabic, is that the standard Arabic is pretty conservative such that MSA, as would be commonly heard on TV or read in the news or other more formal settings, is not very far off the Arabic used in the Quran. So I think most native Arabic speakers would understand the Quran well, but may not be able to fluently speak or write it.

And that many Arabic learners learn MSA along with a dialect, so many second language speakers would probably be able to productively read the Quran as well.