@Entity
public class Book {
    private Long _id;

    private Map<Locale, Title> _translations;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long getId() {
        return _id;
    }

    public void setId(final Long id) {
        _id = id;
    }

    @OneToMany(cascade = CascadeType.ALL)
    public Map<Locale, Title> getTranslations() {
        return _translations;
    }

    public void setTranslations(final Map<Locale, Title> translations) {
        _translations = translations;
    }
}

@Entity
public class Title {
    private String _title;

    @Column(nullable = false, unique = true)
    public String getTitle() {
        return _title;
    }

    public void setTitle(final String title) {
        _title = title;
    }
}