Bug 1732810: Pass file's title to pdfjs download button

So saves as file name rather than 'document.pdf'
Also make en-GB the default english as 'en' is not an lang file for
the pdfjs

behatnotneeded

Change-Id: I320a141ce6c0a0734b9c0e21d6b933f5d99c238f
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>