With the removal of the (standalone) Firefox building code in PR 9566 (a year and a half ago), these files are now completely unused. Hence it doesn't really make sense to keep building them as part of `gulp locale`, and the existing files in the `l10n` folder can also be removed (thanks to version control, they're easy enough to restore should the need ever arise).
This is the PDF.js project output, https://github.com/mozilla/pdf.js Current extension version is: PDFJSSCRIPT_VERSION Taken from upstream commit: PDFJSSCRIPT_COMMIT