\DocumentMetadata{%
pdfstandard=a-4f,
pdfversion=2.0,
lang=en-US,
testphase={phase-III},
}
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{natbib}
\usepackage[spanish,main=english]{babel}
%\usepackage[french,german,polish,japanese,main=english]{babel}
% spanish triggers ! LaTeX hooks Error: Generic hooks cannot be added to '\@lbibitem', if natbib is loaded
% french, german, and various other languages do not trigger this error.
\begin{document}
\section{Introduction}
Some text.
\end{document}