Does German whistleblower law apply to my branch there?

For a German establishment, German law applies, not Dutch law

An establishment in Germany falls in most cases under the German Hinweisgeberschutzgesetz (HinSchG), regardless of what is already regulated in the Netherlands. German law uses the same threshold as the Dutch Protected Disclosures Act: from fifty employees onwards, the obligation to establish an internal reporting channel applies. This count concerns the number of employees working in Germany, not the total number of employees of the entire organisation worldwide. A Dutch employer with a German establishment of fifty or more employees therefore has to deal with two separate laws that apply in parallel: the Dutch one for the Dutch part, the German one for the German part.

Even below that threshold of fifty, the obligation can already apply, namely for employers in financial services, investment firms, and the prevention of money laundering and terrorist financing. This is the same exception as in Dutch law, with slightly different wording. A German establishment that remains below fifty employees on its own therefore does not automatically escape the obligation if the establishment is active in one of these sectors. For the rest, German regulation largely follows the same structure as Dutch law: a receipt of notification and a deadline for information about the assessment, a role for the Betriebsrat comparable to the Dutch works council, and a separate external reporting point at the Bundesamt für Justiz.

Where this is set out: HinSchG and Article 8 of the Directive

The German obligation to establish an internal reporting channel is set out in the Hinweisgeberschutzgesetz, with the threshold and the sectoral exception in § 12 para. 3 HinSchG. This law is Germany's implementation of Article 8 of Directive (EU) 2019/1937, the same EU directive that also forms the basis of the Dutch Protected Disclosures Act — which is why both laws are so similar, yet they are truly two separate national laws that each apply to their own territory.