In this example month names are printed in English even the babel package is loaded with French language.
\documentclass{article}
%\usepackage[pdftex,active,tightpage]{preview}
%\setlength\PreviewBorder{2mm}
\usepackage[frenchb]{babel}
\usepackage{pgfgantt}
\usepackage{lscape}
\begin{document}
\begin{landscape}
\begin{tikzpicture} % optional
\begin{ganttchart}[x unit=2.5mm, time slot format=isodate]{2014-04-28}{2014-07-11}
\gantttitlecalendar{month=name} \\
\ganttbar{Task 1}{2014-04-28}{2014-05-10} \\
\end{ganttchart}
\end{tikzpicture}
\end{landscape}
\end{document}
Babel is not working with pgfgantt?