It achieves zero-shot performance of 11.5% BLEU in French→German and 6.5% BLEU in German→Czech (newstest2013), while  showing comparable or better fine-tuned performance against plain transfer (see also Table 2).
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.