Sort the language nesting mess with polyglossia

Sort the language nesting mess with polyglossia

When using polyglossia, lyx was making a real mess when changing
language inside nested insets. The \begin{language} and
\end{language} commands were not well paired such that they could
easily occur just before and after the start or end of an
environment. Of course this was causing latex errors such that
"\begin{otherlanguage} ended by \end{environment}".
There may still be some cases I did not take into account.