Hi Chouby,
I tested the IF statement by isolating each condition. When isolating isset($polylang), the website returns no posts. Therefore I conclude that somehow there is a problem with the SQL statement. When isolating the other conditions of the IF statement - both the NL and Eng posts are printed, therefore I conclude that these IF conditions are not past and that means, I think, that $join is defined by an earlier statement in the $query.
Regards, Matijs