I'm not sure what you meant here. They did write new lines for the Sultan persona, they just, for some reasons (the voice actor wasn't available?), didn't translate and voice them in Arabic. What they did is just reuse the existing files, even leaving the mistakenly switched "Attacked" and "Declare war" files in the same place.
Because of a lot of cases of back translations we can be sure that the lines are first written by game designers in English and then passed for translation and voicing.
Soon enough Suleiman's new persona will be available and we'll see if this is a pattern or just an unfortunate exception.
That's probably correct, since it seems that for Civ5 they looked for people no further than Canada and USA - Catherine's and Elizabeth's voice actresses are language coaches in Canadian universities and I also read somewhere that Casimir's voice actor sounds like Polish-American, for whom Polish is a heritage language
For Civ6 they actually hired subcontractors in relative countries to find native speaker voice actors and do the recording and that's why the majority of them are good with the language they speak. Except for languages of Mexico (Montezuma's Nahuatl and Lady Six Sky's Mayan), whoever did the recordings in Mexico didn't bother to find native speakers.
That's because it was properly done - by a native speaker in Hawaii. Morningcalm actually contacted him and he wrote about how it was done