4. The system of claim 2 , wherein the operations further comprise identifying one or more groups of phonetic units from among the sequence of phonetic units that form the phonetic representation of the text in the first language; and wherein providing, by the one or more computers, input to the neural network, comprises providing, by the one or more computers, input to the neural network that includes (i) the sequence of phonetic units that form the phonetic representation of the text in the first language, (ii) the language identifier for the first language, and (iii) data indicating the one or more groups of phonetic units.
4. The system of claim 2 , wherein the operations further comprise identifying one or more groups of phonetic units from among the sequence of phonetic units that form the phonetic representation of the text in the first language; and wherein providing, by the one or more computers, input to the neural network, comprises providing, by the one or more computers, input to the neural network that includes (i) the sequence of phonetic units that form the phonetic representation of the text in the first language, (ii) the language identifier for the first language, and (iii) data indicating the one or more groups of phonetic units. 6. The system of claim 4 , wherein the operations further comprise identifying one or more phonetic units that represent stressed sounds in the sequence of phonetic units that form the phonetic representation of the text in the first language; and wherein identifying the one or more groups of phonetic units comprises identifying the one or more groups of phonetic units based on positions of the one or more phonetic units that represent stressed sounds within the sequence of phonetic units that form the phonetic representation of the text in the first language.