I'd like to hear your interpretation, then.
Yes, the Matari slaves were educated in Amarrish and their native language seems to have been suppressed, but again, that's not been the case with the Khanid - they weren't enslaved nor was there any need to suppress their culture as they readily accepted the Amarrian religion.
Modern Amarrish helps little while reading the Scriptures, unless one has a translation at hand. Unless the Evelopedia article got it wrong. And I honestly don't see much interpretational leeway in regard to the Amarrians being willing to translate Scriptures into native languages of foreign cultures. It's the cornerstone of peaceful missionary work and the Amarr never ruled it out as a viable and valuable complement to the forceful reclaiming.
And there is strong evidence that the Khanid people converted peacefully. So, it's quite likely that missionaries taught them the religion in Khanid language, I think.
It's kind'a like how the polish people managed to keep their language and culture alive: They converted to Christianity and joined the Christian western nations under the roman catholic church. Their westernmost cousins didn't manage the transition to Christianity like the polish people did, even though their princes tried to go the same way through adoption of Christianity, they failed to build a Christian Obotrite state.
Also, the fact that Amarrish is the official language of the Empire doesn't mean that there aren't other languages spoken. As Odelya pointed out, in the history of human empires it was rather the rule that they were multi-lingual. The Persian Empires, the Roman Empire, the Empire of Alexander the Great, the Ottoman Empire, the British Empire, the Holy Roman Empire, ... actually, I can't think of a single Empire in the history of humankind where all languages but one were eliminated.
Of course the Amarr would insist that their language will be installed as lingua franca regardless of whether there are also other languages spoken within the Empire.
As an ado, I really can't see the Khanid battalions of the time when the Amarr started to conquer Athra speaking anything but Khanid, while they were fighting at the forefront.I think it's not clear whether they still are but I don't see them turning to a foreign language within a few generations, especially when there is no outside pressure to do so.