Locale is invalid message when trying to pay with a credit card

[Kristof VdB]

Hello,

Since some months visitors can't pay with credit cards on our Prestashop webshop. When someone tries to do so, they get the error 'Locale is invalid'. We don't have this problem with other payment methods.

See screenshot: when you input all data correctly (I've hidden some credit card data for this screenshot), you get the error 'Locale is invalid'. 

I've read that some other customers of our payment provider (Mollie from the Netherlands) had the same issue, but when we contact them, they can't help us.

Maybe interesting to know, we have added nl-nl (Dutch in the Netherlands) and nl-be (Dutch in Belgium) as languages. Could this be the cause of the error? Or is it be-nl or do we better remove the nl-be language? Thanks! Kristof