Some notable neural network based approaches here include the works of (Klementiev et al., 2012;Zou et al., 2013;Mikolov et al., 2013;Hermann and Blunsom, 2014b;Hermann and Blunsom, 2014a;Chandar et al., 2014;Soyer et al., 2015;Gouws et al., 2015).
except for (Hermann and Blunsom, 2014a; Hermann and Blunsom, 2014b), none of these other works handle the case when parallel data is not available between all languages.