committed to tl, closing. thanks!

As of biblatex 3.14, \blx@maplang is only defined \AtBeginPreamble. This breaks with current biblatex.4ht. The following fixes it (biblatex.4ht l.224ff.):

+ \AtBeginDocument{%
\@ifpackageloaded{babel}
{% This is required for languages which are never explicitly selected
% It seems this code was executed also with Polyglossia, where it caused compilation error
\@ifpackageloaded{polyglossia}{}{%
\def\do#1{\blx@lbxinput{#1}{}{}}%
\ifx\@empty\bbl@loaded\else%
\expandafter\docsvlist\expandafter{\bbl@loaded}%
\fi}}{}%
+}