public class RegisterViewModel
    {
        public string Email { get; set; }
        public string Login { get; set; }
        public string Name { get; set; }
    }

    public class ApplicationUser
    {
        public string Login { get; set; }
        public string Password { get; set; }
        public MultilanguageString Name { get; set; }
    }

    public class MultilanguageString
    {
        public string RU { get; set; }
        public string EN { get; set; }
    }

CreateMap<RegisterViewModel, ApplicationUser>().ForMember(dest => dest.Name.RU, opt => opt.MapFrom(src => src.Name));

CreateMap<RegisterViewModel, ApplicationUser>()
    .ForMember(dest => dest.Name, opt => opt.MapFrom(src => new MultilanguageString {RU = src.Name, EN = src.Name}));