[Doitsu]

Originally Posted by XVR

Is there a way to keep non-standard metadata tags in the *.opf file?XVR

This is a known Sigil issue.

@KevinH if you're looking for a test case, here's a minimal Kindle epub dictionary.
When opened with Sigil, the following metadata section is silently stripped:

    <x-metadata>
        <output encoding="utf-8"/>
        <DefaultLookupIndex>russian</DefaultLookupIndex>
        <DictionaryInLanguage>ru</DictionaryInLanguage>
        <DictionaryOutLanguage>en</DictionaryOutLanguage>
    </x-metadata>

Here are the relevant Amazon KDP recommendations for creating dictionaries.