EDIT

DefinedTermBase and KeyTerms are serialized differently:

DefinedTermBase:

{
  'representation_L10n': 'foo',
  'representation_L10n__abbreviatedLabel': 'fo',
  // etc but no 'symbol'
}

KeyTerm:

{
 'message_L10n': 'foo',
 'name': 'foo',
 'symbol': 'sign' // in case of TaxonNodeStatus
}

at least 'message_L10n' should be replaced by representation_L10n