lang attribute in metadata and nav doc

Proposal to create a test to ensure that the lang and xml:lang attributes present in metadata (`content.opf`) and in the navigation file (TOC) digested by the reading application are respected and correctly passed to the AT.

Ensuring that the lang and xml:lang attributes are present and correctly passed on to users is essential for accessibility. These attributes specify the language of the content, which is crucial for users who are visually impaired or have reading disabilities. Screen readers use the lang and xml:lang attributes to determine the language to use when reading the content to the user. If these attributes are missing or incorrect, it can result in a poor user experience for the user, making the content difficult or impossible to understand.