Add last_edit for translations to json

Same as in pull #1914 but this time for translations.

See e.g. https://developer.mozilla.org/en-US/docs/Web/CSS/color$json?expand=1 
There is an array of translations available and it would be really useful to have the last_edit information here, too. Would allow us to test with a single call if
a) a translation is available and,
b) whether the translation is older than the English document.