Yes, the Galaxy A26 supports Samsung's Dual Audio feature for connecting two Bluetooth audio devices simultaneously. To activate it, first pair two Bluetooth devices, then scroll down the quick panel, tap Media and select both devices. Note that this feature is available on devices compatible with Bluetooth 5.0 and One UI 2 or higher.
No, the model does not support 1 dedicated SIM and 1 dedicated SIM plus SD at the same time. It has a dedicated SIM 1 slot and a hybrid slot for second SIM or MicroSD (up to 2 TB). This is confirmed by Samsung's official specifications for the SM-A266BZKBEUE.
Yes, the mobile can be used with German SIM cards. It supports dual SIM with nano SIM cards and eSIM, which are common SIM card formats in Germany. This means it can be used with German mobile phone providers without any problems.
This device does not have a 3.5 mm headphone jack. Nor is it possible to use a USB-C to jack adapter to connect wired headphones, as the device does not support this feature. As for inserting an SD card, the information available does not mention a dedicated or hybrid (SIM/SD) port for an external memory card, which suggests that it is not possible to add an SD card to this model. So we'll have to make do with the 128GB of internal storage space.