Each footnote in an ESEF file is required to either possess or inherit the attribute 'xml:lang' with a value corresponding to the language of content of at least on textual fact in the XBRL document. Furthermore, every footnote relationship is also required to have at least one footnote in the language of the report in question.
If you receive the message Violation: “footnoteInLanguagesOtherThanLanguageOfContentOfAnyTextualFact” when validating an ESEF report, it means that the report in question violates the rules specified above.