For all transfer learning setups, we learned byte pair encoding (BPE) (Sennrich et al., 2016) for each language individually with 32k merge operations, except for cross-lingual encoder training with joint BPE only over source and pivot languages.
to other NMT transfer scenarios (Zoph et al., 2016;Nguyen and Chiang, 2017;Kocmi and Bojar, 2018), this principle has no language mismatch between transferor and  Figure 2: Step-wise pre-training.