I always travel with a letter from my endo specifying the equipment and supplies I am carrying. In roughly 40 years of flying internationally, I have only ever needed to show it once – but with Italian police pointing submachine guns at me, I was sure glad I had it.
I’ve travelled with my insulin at room temp in warm climates for up to a month, and it’s been fine. But if you’re concerned, check out Frio bags, which are popular. Do not keep insulin in a hotel-room fridge, which may freeze it. You don’t need a prescription to buy insulin in Germany, but make sure what you use is available there. Novolog is, but I think Tresiba was taken off the market a year or so ago. It may be back by now.
Most Europeans speak good English, but if you’re going off the beaten path, you may want to carry the contact info of an English-speaking endo or family doctor, because the local doctor (and if you’re in a remote place, there my not be a local doctor) may not speak very good English, and in a medical emergency, you want clear communication. You can find names and numbers online, or the US embassy or consulate closest to where you’re headed may be able to help.
It was said above but it’s worth repeating: carry all your medical supplies in your carry-on baggage. Triple-check your baggage allowance for your airline and your class. With many Star Alliance airlines you can board with two bags in economy, which should be ample. (If I had a dollar for every time I’ve had to watch someone repack their bags because they thought everything was carry-on …)
Remember to take your Dexcom charger adapter.