I wanted to take a go at this, but I'm stuck. What I did was to separate the "attribution" string in /data/background_images.json into logical parts, take a look here.

I know what needs to happen in the next steps, but I'm unfortunately not familiar enough with Javascript/jQuery to do it myself. Anyways, my plan was to:

Add to /i18n/de/i18n.json a string like:

"bgattribution": "%{author},  „%{title}“, %{license}"

(And the same for /i18n/en/i18n.json with English quotation marks, of course.)

Then, in /js/background.js you need to somehow call the correct i18n file and supply the bgattribution string with the correct substrings fro background_images.json.