I tried different options in \definedescription and compared LTR and RTL behavior.  There are certain inconsistencies/bugs in the RTL setting.  Attached you can see a TeX file and the resulting PDF, trying different options.  The first two pages consider different parameters in the LTR setting and then the same come in the RTL environment.

In the following I use alt instead of alternative.

Here are the issues I noticed:

1) Why isn't the head text consistently mirrored in the RTL environment?  It happens only in alt=serried.

2) Option hang=2 produces a strange result for RTL.  The hanging part appears on the opposite side of the head text, which somehow pushes the head text outside the text area.

3) Option alt=top for RTL still puts the head text on the left-hand side.

4) I expect the LTR and RTL versions of the same settings to mirror one another, but they do not.   Although LTR/alt=left and RTL/alt=right look pretty similar (and so do LTR/alt=right and RTL/alt=left), but these pairs do not mirror one another.  In particular, there is some space to the right of the head text in LTR/alt=right, while there is none to the left of LTR/alt=left.  The same thing shows in the margin alternatives where the distance between the head text and the body is different in RTL and LTR.

—MHB