You can chenge the direction by modifying the skin

html{
direction|: rtl
}

putting this in the stylesheet of your skinn will change the direction.

as for the menu, I advice you to use the SolPart menu instead of the DNNMenu as the later one has a problem with RTL, also you will notice that you will have to do some modifications to the core login and registration, since they are not designed i a way that support rtl.

Hope this helps.