Some common signals to learn bilingual embeddings come from parallel (Hermann and Blunsom, 2014;Luong et al., 2015;Levy et al., 2017) or comparable corpora (Vulić and Moens, 2015a;Søgaard et al., 2015;Vulić and Moens, 2016), or lexical resources such as WordNet, ConceptNet or BabelNet (Speer et al., 2017;Mrksic et al., 2017;Goikoetxea et al., 2018).
these sources of supervision may be scarce, limited to certain domains or may not be directly available for certain language pairs.