I believe the dictionaries are linked to the firmware rather than the device, and as Kobo has done a fairly good job keeping their devices all on the same firmware they should all share the same dictionary.
Worth noting that neither of the two English dictionaries available for the Kobo on MR were able to define atrocities. Plurals generally don't get unique entries in dictionaries. Could the digital look up be made smarter so it scanned some more of the dictionary file? Maybe. But I think you'd run the risk of it returning words that were just part of the definition.
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