For a start, alt texts could be added like this, to the image.

Note: remember to register the component in a composer

using System;
using Umbraco.Core.Composing;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using System.Collections.Generic;

namespace Website.Services
{
    public class MediaType: IComponent
    {
        private readonly IMediaTypeService _umbracoMediaTypeService;

        private readonly IDataTypeService _dataTypeService;

        private readonly ILocalizationService _locationService;

        public MediaType(IMediaTypeService mediaTypeService, IDataTypeService dataTypeService, ILocalizationService locationService)
        {
            _umbracoMediaTypeService = mediaTypeService;

            _dataTypeService = dataTypeService;

            _locationService = locationService;
        }

        public void Initialize()
        {
            InitialiseImageMediaType(); 
        }

        public void Terminate()
        {
            throw new NotImplementedException();
        }

        private void InitialiseImageMediaType()
        {
            IMediaType imageType = _umbracoMediaTypeService.Get("Image");

            if (imageType != null)
            {
                IEnumerable<ILanguage> languages = _locationService.GetAllLanguages();

                foreach (var lang in languages)
                {
                    //Add alt text for each language
                    var TextstringDataType = _dataTypeService.GetDataType("Textstring");
                    PropertyType myProperty = new PropertyType(TextstringDataType, "alt_" + lang.IsoCode);
                    myProperty.Name = "Alternate text for " + lang.CultureName;
                    imageType.AddPropertyType(myProperty, "Image");
                    _umbracoMediaTypeService.Save(imageType);

                }
            }
        }  
    }
}

This gives me these alt texts, depenind on my langs