What is the issue?
PDF/UA > Basic requirements > Natural language > Natural language of alternate names of form fields > Natural language of an alternate name of a form field cannot be determined
What does it mean?
There is no document language determined. As a consequence: There is no language determined for the quickinfo of a form field.
How to fix it?
Use the task pane Settings 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.