It seems it is for the metadata, according to Example 1 at your link:
Example 1
Quote:
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The xml:lang attribute is set to English on the package element to ensure the metadata in the package is correctly interpreted.
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I will add this for a future release and assume the primary (first dc:language) value is what the metadata is written in.
It might be a while as I am moving the entire Access-Aide plugin gui to be PySide6/PyQt5 Qt based as Tk has many flaky issues with svgs and with the image table for adding alt values.