Not sure if this feature has been requested yet or not (since the XSLT files can be edited more easily outside of the core application), but there are a lot of English-only labels in the transformation files, e.g. https://github.com/archivesspace/archivesspace/blob/master/stylesheets/as-ead-pdf.xsl#L171
If desired, multiple translation options could be added for these files, as well. Easy enough to extract the English-language defaults already used here, but it would be quite a bit more work to create the translations (es, fr, and ja).
Just thinking of this while sketching out what we'd need to do to share our EAD3-to-PDF/UA process.