In order to get the engine to properly fallback to an Arabic supported TTF you’ll need to translate (“Slate”, “FallbackFont”) in the archive. The translation should be the name of a TTF font under Engine/Content/Slate/Fonts.
For example, the default translation for this is “DroidSansFallback”, but the Korean archives translate it to “NanumGothic”.
I realize this is pretty terrible solution. In the near future we’ll be implementing something similar to composite fonts which will better resolve problems like this.