Automatic Machine Translation Evaluation in Many Languages via Zero-Shot Paraphrasing

We propose the use of a sequence-to-sequence paraphraser for automatic machine translation evaluation. The paraphraser takes a human reference as input and then force-decodes and scores an MT system output. We propose training the aforementioned paraphraser as a multilingual NMT system, treating paraphrasing as a zero-shot "language pair" (e.g., Russian to Russian). We denote our paraphraser "unbiased" because the mode of our model's output probability is centered around a copy of the input sequence, which in our case represent the best case scenario where the MT system output matches a human reference. Our method is simple and intuitive, and our single model (trained in 39 languages) outperforms or statistically ties with all prior metrics on the WMT19 segment-level shared metrics task in all languages, excluding Gujarati where the model had no training data. We also explore using our model conditioned on the source instead of the reference, and find that it outperforms every quality estimation as a metric system from the WMT19 shared task on quality estimation by a statistically significant margin in every language pair.