The Alt tag process in Prismic gives you various options for adding alt tags in different languages and documents so you can improve your SEO and rank better on Google's search results.
Adding unique Alt texts to images
Once you add your image in the document you can add and edit your image alt text just below the image. This also means that if you have the same image across multiple documents or multiple locales you can add a unique alt tag for each instance of the image.
So for example, if you have the same image on an English language document and a French language document, then you can have a different image alt tag in each locale without affecting the original image in the image library.
Adding the Image alt in the media library
When you add your image to the library you can add default alt text for your image. After filling in the default alt text, when you add that image to a document, the alt text will automatically fill with the default value. Note that this will only automatically fill when you add the image to a document.
The default alt text will not automatically update
As mentioned above, when you add an image to your document, it will automatically fill with the default alt text set in the Media Library. However, if you then make a change to the default alt tag in the Media Library, this will not automatically update the alt text everywhere that image is used. In order to use the new default text, you will need to go to the document where that image is used, remove it, then re-add it to see the new alt text.