Fix issues with active language selection.

* Add CSRF protection.
* Fix remote file read (language=../path/to/any/file.txt).
* Validate selected language option.
* Return error message on error.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5720 2fea6a4d-a838-0410-917b-93a53c48e9d2