SELECT a.codarta,a.codartb,a.descarta,a.descartb,a.notizi e
FROM articoli as a, trad_articoli as t
WHERE a.codarta = t.codarta
AND t.lingua = 2
AND a.codarta IS NULL

SELECT a.codarta,a.codartb,a.descarta,a.descartb,a.notizi e
FROM articoli as a
LEFT JOIN trad_articoli as t
ON a.codarta = t.codarta
WHERE t.lingua = 2
AND a.codarta IS NULL

Provale, sono equivalenti.