Added extra english entry for babel; English is needed by example.bib.

example/mu/example.dtx

@@ -55,7 +55,7 @@
   main=english, %% By using `czech` or `slovak` as the main locale
                 %% instead of `english`, you can typeset the thesis
                 %% in either Czech or Slovak, respectively.
-  german, russian, czech, slovak %% The additional keys allow
+  english, german, russian, czech, slovak %% The additional keys allow
 ]{babel}        %% foreign texts to be typeset as follows:
 %%
 %%   \begin{otherlanguage}{german}  ... \end{otherlanguage}