Fix bug #10685

Fix bug #10685

Make sure to properly nest \begin{lang} and \end{lang} tags even
when no language package is selected. In this case, LyX assumes
that babel is being used, so the language names might be wrong
if the user arranged for using polyglossia in the preamble.
Nevertheless, we assure that the produced output is syntactically
correct, so that by adding proper preamble code a correct output
is still possible.