good question! we'll need to fix that.
Withdrawing from groups
I don't think you can. I think you'll have to rejoin the group, delete the notice and withdraw from the group again.
By the way how many languages do you speak? I've noticed three.
Only one, but like most people I've picked up the odd word here and there from several others. 
And you're perfectly correct. I had to rejoin the group temporarily.
I recently withdrew from a group but, before going, had ticked at least one "track this forum" box. Today I received a notice in the "New Comments" box on my home page directing me to that forum. Since I no longer have access to the group that notice cannot be answered or deleted.
How is it possible to delete this notice (without rejoining the group, deleting it, then again withdrawing) please?