[FIX] product: find products of subcategories

When searching products for a specific category, display the products of the whole category subtree.
[FIX] website_mail: avoid error when message.body is empty
[FIX] point_of_sale: account selection for payments

Registered payment uses the partner receivable account. As this field is
a property field, it will select different accounts based on the user that
registers the payment (in multicompany).
Should use the company of selected journal instead of the one of the user.
[FIX] payment_buckaroo: dynamic lang & test params

test flag is set according to acquirer environment
lang is set according to partner language
[FIX] payment_buckaroo: api expects iso culture as language param

IT looks like this is the code we use in partner lang, by replacing the '_' (underscore) by '-' (dash)