Also track local language switches

Also track local language switches

Until now this was not done for essentially two reasons. The first
one is that local switches are used for short text insertions, so that
they are unlikely crossing environment boundaries. The second one
is that if we have to close a language at the end of an environment
we would be missing the right termination command. As this last
issue can be overcome by simply storing in the stack the current
nest level with a sign denoting the kind of switch, there is no
reason anymore not to track also local languages switches.
Also factor out some commonly used constructs in order to improve
readability.