The Moodle plugins database has just accepted Gordon Bateson's awesome Essay (auto-grade) plugin.
The "Essay (auto-grade)" question type awards a provisional grade as soon as the student submits an essay. The provisional grade is based on the number of words, sentences and target phrases (also common errors) that are present in the essay.
In addition, this question type allows the teacher to specify "grade bands" which can be used to create a non-linear grading scheme.
At any time, the teacher can override the automatically generated grade by giving a manual grade and feedback as in the standard "Essay" question type.It already has a Mexican Spanish translation made in AMOS and a Moodle Docs page with a couple of illustraded sample questions.