The ElasticPress' "Documents" feature, responsible for indexing attachment contents, needs an Ingest Pipeline to work properly. ElasticPress creates that in the Documents::create_pipeline() method, hooked to the ep_cli_put_mapping and ep_dashboard_put_mapping actions.
I did not have the chance to test it myself but based on some reports we are getting, it seems WPML ElasticPress needs to implement the creation of that pipeline for additional languages/indices as well.
Happy to provide any additional info if needed. Thanks!