\documentclass{article}

\usepackage{polyglossia}

% If you uncomment this line, the display date will
% appear in unlocalized form (numeric month)
% \usepackage[useregional]{datetime2}

\setdefaultlanguage[variant=american]{english}

\usepackage[useregional]{datetime2}

\begin{document}

% I want this to appear as: March 28, 2014 when
% \setdefaultlanguage is English, or mars 28, 2014
% when \setdefaultlanguage is French
%
% I'm using datetime2 in a document class, but
% definining document languages in the document
% that uses it, so the language is not set until
% after datetime2 is loaded.
%
% I need a method to force datetime2 to re-scan
% the list of tracked languages and check for
% corresponding installed modules.
\DTMdisplaydate{2014}{3}{28}{-1}

\end{document}

\endinput
