I recently posted about my first impressions for Scrivener on iOS and how much I love it. I’ve continued my love affair with the software, but have been hitting one issue for which I now have a resolution; the spell-checker language kept using American English.
I searched the Scrivener for iOS app settings for a language or dictionary option, but couldn’t find one. I checked the official knowledge base, but there is very little there on the iOS version. Finally, I found a response in the discussion forums that answered my questions.
It turns out, the Scrivener spell-checker uses the language dictionary associated with the iOS keyboard selected. You can find keyboard settings in the settings app under General > Keyboards.

Within the keyboards section, there are two places to set the keyboards available: “Keyboards” controls the software keyboards available; “Hardware Keyboard” (as the name suggests) controls the settings for physical keyboards you attach.
By default, I had three software keyboards configured; Australian, emoji, and American. I use the Apple keyboard cover, and am constantly pressing the little globe button that changes the language — it seems that act key changing the spellchecker language to American English. I have since deleted the American version and since then the spellchecker appears to be correctly checking against the Australian English dictionary.