Add translations for ProductMedia alt text

Problem

Saleor supports translations for many catalog fields, including product names, descriptions, SEO fields, categories, collections, attributes, attribute values, and variants.

However, ProductMedia.alt currently has only one global value and does not support translations.

This makes it difficult to build a multilingual storefront with localized image alt text. Image alt text is important for accessibility, SEO, and localized user experience.

Discord Discussion Link

https://discordapp.com/channels/864066819866624010/1523694589767192666

General Assumptions

ProductMedia.alt should support translations in the same way other catalog entities do.

For example, it should be possible to query localized alt text for product media:

product(id: "...") {
  media {
    id
    alt
    translation(languageCode: DE) {
      alt
    }
  }
}

This would allow storefronts to display the correct image alt text depending on the selected language.

API Changes

No response

Database Changes

No response

UML Diagrams

No response

To Do List

No response

Testing Requirements

No response