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 tab "Settings" in axesPDF QuickFix and enter the language code into the field "Language": for example "en-gb" for British English, "en-us" for US Englisch or "de" for German.