must I inform them which countries I do business with

Country risk is a standard part of customer due diligence

Yes, in practice the bank asks which countries you do business with, and they are entitled to do so. This falls under the customer due diligence required by the Money Laundering and Terrorist Financing Prevention Act (Wwft). The bank must assess risks, and countries with an elevated risk of money laundering or terrorist financing are a fixed criterion in doing so. Often the questionnaire does not ask about each country separately, but about countries outside the EU/EEA, or specifically about countries on a risk list. Read the question literally therefore: "foreign customers" can in practice mean "customers outside the EU" or "customers in high-risk countries", and that makes quite a difference in what you need to list.

What you concretely provide depends on how the question is formulated: a list of countries, a percentage of turnover per region, or only a confirmation that you do not do business with countries on a sanctions or risk list. The latter also relates to the question or whether the enterprise or any of its directors is on a sanctions list, as country risk and sanctions risk are often mentioned by banks in the same breath, but they are two different tests. If you do not have a ready-made overview of your customer countries, a reasonable approach is to work from your invoicing or CRM data, and to mention that separately if the bank asks for a source reference.

Basis: AML client due diligence, not the Commercial Register

The obligation to report countries of business relations does not come from the Trade Register Act 2007 — which only establishes who the enterprise is and who belongs to it, see also who are the ultimate beneficial owners of the company — but from the customer due diligence that institutions such as banks must carry out under the Wwft. This law requires the bank itself to assess risks, and country risk is a recognised factor in doing so; the enterprise provides the data that the bank needs to be able to make that assessment.