add a language tag to PDF metadata export

[Erwan-le-Gall]

label: accessibility

When I create a Docs document

Then I should be able to choose the langage of the document OR the default should be (fr-FR) French-France.

Why is this important: because disabled persons (blind…) use a screen reader and voice synthesis. Pronunciation is guided by this value. Ie the word "mobile" has a different pronunciation in English and in French.

reference: https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf#page=619

[Erwan-le-Gall]

ping @cyberbaloo

[Erwan-le-Gall]

@virgile-dev @AntoLC

I think the answer is in:

language – Sets PDF default language