handle multi-language epub/pdfs

[elstoc]

epub and pdf files in the translated site contain a mixture of English and translated pages. We will need to change these so that they are generated per-language (or just English for now).

[elstoc]

For now I've moved the translation generation step in the CI until after the epub and pdfs have been generated but this will need to be addressed when the translations are complete.

[paperdigits]

I think we might need a little piece of config for each language to make that particular language the default...

[paperdigits]

the PDF renders using our current setup, but the images do not. The links to the images have the language code in the URL, but they should all point to the "English" image.

[elstoc]

We fixed this for the main site some time ago so it should be simple enough to port that fix to the pdf. The epub might be more problematic, since we need the images to be physically present in the rendered epub.

I think we'd rather have a pdf/epub for each language rather than a single multilingual pdf/epub (or I would anyway).