When I create a new device definition in AVD Manager, I can create a new AVD with this device definition. If I close and reopen AVD Manager, the device definition is lost and the AVD created gets "repairable" status.
This seems to be a locale specific bug. We are discussing it on the mailing list: https://groups.google.com/forum/?fromgroups=#!topic/adt-dev/nfJvhxO8iJk
As you pointed out, a temporary fix is to create the device definition, then edit ~/android/devices.xml
and replace commas with dots.