try this query:
Select ps_product.reference As ART, ps_product.supplier_reference As ARTHersteller, ps_product.ean13 As EAN,
ps_product.quantity As Anzahl, ps_product.price As Preis, ps_product.wholesale_price As WholesalePrice,
ps_product.location As Standort, ps_product.active As Aktiv, ps_supplier.name As Lieferant,ps_product_lang.name
From ps_product
left Join ps_category On ps_category.id_category = ps_product.id_category_default
left Join ps_supplier On ps_supplier.id_supplier = ps_product.id_supplier
LEFT JOIN ps_product_lang ON ps_product.id_product=ps_product_lang.id_product