add a language tag to DOCX exported document

[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 (and not en-NL English of Netherlands %-( ).

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.

[Erwan-le-Gall]

ping @cyberbaloo