UPDATE table based on the same table
This is rather risky business, and I can understand why. It has to do with the way MySQL processes subqueries. I wrote about it back on Feb 22, 2011 : Problem with MySQL subquery
Performing JOINs involving SELECTs and subquery SELECTs are OK. On the flipside of things, UPDATEs and DELETE can be a rather death-defying adventure.
SUGGESTION
Try refactoring the query so that it is an INNER JOIN of two tables
UPDATE
    products_description pd INNER JOIN products_description pd2 ON
    (pd.products_id=pd2.products_id AND pd2.language_id=1 AND pd.language_id&lt;&gt;1)
SET pd.products_seo = pd2.products_seo;

Give it a Try !!!