<texcode>
% engine=luatex\startenvironment ara-sty
\mainlanguage[arabic]
\setcharactermirroring[1]
\stopenvironment
</texcode>
Here is some description:
<texcode>
% engine=luatex
</texcode>
This tells ConTeXt to use [[luatex]] as TeX engine, this mandatory for this example to work.
<texcode>
Now, lets try a "Hello World" example:
<texcode>
% engine=luatex \input environment ara-sty
\starttext