Add another constructor to scoped_restore_current_language

Add another constructor to scoped_restore_current_language

While working on something else, I noticed that this is relatively
common:

  scoped_restore_current_language save;
  set_language (something);

This patch adds a second constructor to
scoped_restore_current_language to simplify this idiom.

Reviewed-By: Tom de Vries <tdevries@suse.de>