[KevinH]

Access-Aide is a plugin that is meant to help with just that. It properly fills in empty titles inside head elements, it will properly add xml:lang to the opf package tag, and lang and xml:lang to html tags, and will give show you a two column gui table showing a thumbnail of every image side by side with the current alt text value. If no alt text value exists for an image, it will try to access the image's own metadata to extract one.

But if the image's metadata is missing, then you still have to hand type in a proper alt value into that table.

Upon table completion, any changed alt values are updated in the text by the plugin.

You might want to give it a try as creating a table first before trying to apply it takes as much time as directly editing the alt in the gui table and let the plugin apply it.