Various improvements for Spanish translation (build scripts and HTML, not actual translation)

[gjvnq]

I made various fixes to the UI and the build scripts to better support multiple languages.

I didn't translate anything other than short passages necessary for the UI.

[Twipped]

😮 Amazing! Thank you! Putting a lang entry into the metadata didn't even occur to me, of course we need that.

[gjvnq]

Talking about metadata, we should probably look into adding Dublin Core metadata for compatibility with Zotero and other software.