\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[english,spanish]{babel}
\addto{\layoutspanish}%
{\renewcommand{\labelenumii}{\arabic{enumi}.\arabic{enumii}}}

\begin{document}

\begin{enumerate}
	\item bla
	\item bla
	\begin{enumerate}
		\item Bla
		\item Bla
	\end{enumerate}
\end{enumerate}

\end{document}
