Poland and Estonia.
No, but in every one of those cases English is the way to bet.
> What about a Belgian company from Wallonia and a French partner? Or Luxemburg and Wallonia?
Almost certainly French because it’s the working language of both companies. If people share a native language they’ll use that. Otherwise the default is English. It’s not certain that they’ll use English but Siemens doesn’t expect its international partners to have German speaking partners for dealing with it; it does expect its executives to speak English.
Just as science has a lingua franca so does business. It is English.
Once I had a business meeting in Switzerland. It was all in English (for me, the American), but at one point they were speaking German to each other. And later in the hallway, they switched to speaking French with the lawyer who was there with me. And at lunch in the restaurant attached to the office, the Swiss German host ordered his food in Italian.