[Renate]

I have no idea what the problem is.
Maybe your Word is doing stuff. Just use any normal text box.

If you start empty and hit something in Arabic then it goes into RTL mode.
If you hit something first in English it stays in the default LTR.

(The Arabic word I use is what you get when you hit "qwerty".)

The first is "qwerty Hello qwerty Goodbye".
The second image is "Hello qwerty Goodbye qwerty".
They behave as I'd expect.