change labelText direction to RTL in InputDecoration() flutter

Can be used textAlign

            TextFormField(
              textAlign: TextAlign.right,
              decoration: InputDecoration(
                hintText: 'ادخل تفاصيل الكتابة الخاصة بك',
              ),

Simply use Directionality:

new Directionality(
    textDirection: TextDirection.rtl,
    child: TextField(
     textAlign: TextAlign.right,
     controller: _textEdittingControler_bookName,
     autofocus: true,
     decoration: new InputDecoration(
                labelText: "افزودن کتاب",
                hintText: "نام کتاب را وارد کنید"
                ),
     )

Directionality's docs