>> Will your proposal make it harder to type in such languages?
>
> Yes, in some cases.
>

I don't see why they would become harder to type in in any way.  The 
directionality characters would be highlighted, that's all (and it would 
be easy to add a toggle command to (un)highlight them on demand, see 
attached).  This functionality is orthogonal to the one you propose AFAIU, 
both could coexist.