I have a solution but I am sure there must be a better way:
select a.idLANG, decode((select b.idLANG from langExpression b, langue c where b.idExp = '2' and c.idLANG = b.idLANG and b.idLANG = a.idlang), null, 'false', 'true') as ok
from langue a
Thanks
Stroy