What is the issue?
PDF/UA > Basic requirements > Natural language > Natural language of outlines can be determined > Natural language of an outline item cannot be determined
What does it mean?
There is no document language determined. As a consequence: There is no language determined for outline items.
How to fix it?
Use the task pane "Settings" in axesPDF QuickFix and enter the language code into the box "Language": for example "en-GB" for British English, "en-US" for US English or "de-DE" for German.