Thanks jhowell! This is encouraging. I'll see if my regexp skill is sufficient to make a workable lookup dictionary (I'm not looking for perfection).
The OPF file I get from the KindleUnpack plugin, taking the Spanish dictionary provided by Amazon for free, does not contain several lines that I would have expected from the Amazon guidelines you referenced. I re-downloaded this dictionary a couple of days ago to make sure I had the latest version and KindleUnpacked it.
As you can see, it doesn't have '<x-metadata>' around <DictionaryInLanguage>es</DictionaryInLanguage>. In any case, getting the OPF file right will be the easiest part of this endeavour.
Code:
<?xml version="1.0" encoding="utf-8"?>
<package version="2.0" xmlns="http://www.idpf.org/2007/opf" unique-identifier="uid">
<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
<dc:title>Real Academia Espaņola el Diccionario de la lengua espaņola</dc:title>
<dc:language>es</dc:language>
<dc:identifier id="uid">737551192</dc:identifier>
<dc:creator>Real Academia Espaņola</dc:creator>
<dc:contributor>calibre (4.13.0) [https://calibre-ebook.com]</dc:contributor>
<dc:publisher>Amazon Dictionary Account</dc:publisher>
<dc:source>calibre:e1734abb-ad6d-48a2-8012-5bd13419fd53</dc:source>
<dc:date opf:event="publication">0101-01-01 00:00:00+00:00</dc:date>
<meta name="cover" content="cover_img" />
<meta name="output encoding" content="utf-8" />
<!-- BEGIN INFORMATION ONLY
<meta name="Cover ThumbNail Image" content="Images/image32086.jpeg" />
<meta name="ASIN" content="e1734abb-ad6d-48a2-8012-5bd13419fd53" />
<meta name="Updated_Title" content="Real Academia Espaņola el Diccionario de la lengua espaņola" />
<meta name="Creator-Minor-Version" content="9" />
<meta name="StartOffset" content="98947" />
<meta name="Creator-Build-Tag" content="0730-890adc2" />
<meta name="MetadataResourceURI" content="kindle:embed:0001" />
<meta name="Creator-Software" content="202" />
<meta name="HasFakeCover" content="0" />
<meta name="cdeType" content="EBOK" />
<meta name="Metadata-Record-Offset" content="0" />
<meta name="Creator-Major-Version" content="2" />
<meta name="Embedded-Record-Count" content="2" />
<meta name="Content-Language-Tag" content="es" />
<meta name="Creator-Build-Number" content="0" />
END INFORMATION ONLY -->
</metadata>
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<reference type="text" title="Comenzando" href="Text/part0005.xhtml#aid-4OIQ2" />
<reference type="toc" title="Contenido" href="Text/part0000.xhtml#filepos166" />
<reference type="cover" title="Cover" href="Text/cover_page.xhtml" />
</guide>
</package>
NOTE: When I first posted, I included the wrong OPF. That mistake was corrected after about 10 minutes.