No. I meant "I can’t." Let me explain.
I usually use Japanese input, Hiragana and U.S.
and I found I couldn’t type Arabic letters with Arabic input.
Trying other inputs, I found I couldn’t type Cyrillic in addition to Arabic and had no problem typing any Asian letters such as Korean, Hindi, Thai.
And I also found I got to be able to type Arabic only after choosing Arabic input after Turkish until switching US or Japanese.
I turned Siri off but it didn’t solve the problem.
It seems the same as Imotov reported except that his/her workaround doesn’t work in my environment.
My workaround is: before switching to Russian, switch to Turkish and then switch to Russian.