The epub3-itizer plugin is an output plugin. You need to load the file it creates into Sigil. It does not not change your current file. And it definitely updates the package tag to "3.0".
My guess is you kept using the file you ran epub3-itizer on.
According to Daisy -
Quote:
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EPUB 2 does not allow a global language declaration using the xml:lang attribute on the package element. You must declare an xml:lang attribute on every metadata tag. With a global language declaration on the package element, you only need to override that declaration if metadata is written in another language.
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I will make sure to modify Access-Aide to only add the xml:lang to the package tag on epub3, not epub2.
But given how long epub3 has been out now, and that epub2 is long deprecated, no newly created epub2s should be being created anymore. An epub3 made backwards compatible with epub2 can do everything an epub2 can do and more.
You might want to move to epub3.