A multilingual model is usually trained by maximizing the likelihood over training sets D i,j of all available language pairs S. That is: where we denote L j θ (x i , y j ) = log p(y j |x i , j; θ).
the target language ID j is given to the model so that it knows to which language it translates, and this can be readily implemented by setting the initial token y 0 = j for the target sentence to start with.