Using non-numerical ranges in Tikz bar plots

Question

I am trying to draw a horizontal bar plot using Tikz and I would like to use non-numerical tags for the x axis.

More precisely, I have the following diagram:

and I would like to change the numeral tags 1, 2, 3, 4, 5 and 6, by the usual language levels A1, A2, B1, B2, C1, and C2.

The code is

\documentclass{article}
\usepackage{pgfplots, pgfplotstable}
\usepackage{tikz}               
\usetikzlibrary{shapes, backgrounds,mindmap, trees}

\begin{document}

\section{Spanish Level (CEFR)}
\begin{center}
\begin{tikzpicture}
    \pgfplotstableread{ % Table macro
      Label      First   Second 
      Speaking     5          1 
      Listening    3          1
      Reading      2          2
      Writing      3          1
   }\datatable

   \begin{axis}[
     axis line style={draw=none},
     enlargelimits=.1,
     xlabel={CEFR level},
     xbar stacked,   % Stacked horizontal bars
     xmin=0.5,       
     tick style={draw=none},
     y=-0.5cm,
     bar width=0.4cm,
     enlarge y limits={abs=0.45cm},
     ytick=data,
     yticklabels from table={\datatable}{Label},  % Get the labels from the Label column of the \datatable
     legend style={at={(.5,-.5)}, anchor=north}
   ]
     % Plot the "First" column against the data index
     \addplot [fill=red] table [x=First, y expr=\coordindex] {\datatable};
     \addplot [fill=red!20!blue] table [x=Second, y expr=\coordindex] {\datatable};
     %\addplot [fill=red!80!yellow] table [x=Third, y expr=\coordindex] {\datatable};
     \legend{certified, de facto}
  \end{axis}      
\end{tikzpicture}
\end{center}
\end{document}