not specifically related to WooCommerce 2.6 version; this happened already before.
I first thought it was a WPML problem, and logged a ticket with them.
They took the time to disable all plugins (and so did I), to no avail.
When we switched to TwentySixteen (and with all plugins active) the Dutch translation of the checkout works properly.
Not sure why it is only Dutch; the French and english versions work ok.
Visible symptoms : the checkout page remains greyed out, and when you fill in the address fields, you can notice a very quick flicker of the “order” button, but you can never complete the checkout.