Update row with select on same table

For MS SQL you can use a join to do this, it might not work with other databases though:

UPDATE t1
SET t1.text = t2.text
FROM table1 t1
INNER JOIN table1 t2 ON t1.id = t2.id 
WHERE t1.TEXT IS NULL AND t2.LANG = 'EN'

Sample SQL Fiddle

UPDATE TABLE
SET TEXT = (SELECT TOP 1 text WHERE id = table.id AND LANG ='EN' AND TEXT IS NOT NULL) --TOP 1 INCASE YOU HAVE DUPLICATES
WHERE TEXT IS NULL

You don't specify the database. The following is standard SQL:

UPDATE t
    SET TEXT = (SELECT text
                FROM t t2
                WHERE t.id = t2.id AND LANG ='EN' AND
                      TEXT IS NOT NULL
               ) 
    WHERE TEXT IS NULL;

In the event of duplicates, the following should work:

UPDATE t
    SET TEXT = (SELECT max(text)
                FROM t t2
                WHERE t.id = t2.id AND LANG ='EN' AND
                      TEXT IS NOT NULL
               ) 
    WHERE TEXT IS NULL;

EDIT:

Of course, not all databases support all ANSI standard functionality. In MySQL, you would use a join instead:

UPDATE t JOIN
       (SELECT id, max(text) as text_en
        FROM t t2
        WHERE LANG ='EN' AND TEXT IS NOT NULL
       ) ten 
       ON t.id = ten.id
    SET t.TEXT = ten.text_en
    WHERE t.TEXT IS NULL;