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The D400 series will be using SDK 2.0 for Linux and windows. This SDK has already been out for a while, you can download it if you want and start trying it out.
Marty G did a great post regarding the SDK 2.0 and documentation.
To answer your question, since the SDK does support all this languages you can use whatever is more convenient for you. C++ is what is most commonly used and eventually you will find more community help in C++. ROS is not a bad option either thanks to the SLAM modules.
From my personal experience, working within the ROS framework will help you write a code that is as modular as it gets. It is very good practice to use it, and it will help you write code that you can easily share with others via github, since it's relatively easy to work with, especially when working on the Intel Euclid.
Within the ROS framework, you can use either C/C++ or Python, for the most part. If you don't have previous experience in coding, C or Python will probably be the easiest to use, and assuming you're going to want to use libraries for object detection and the like, you are probably going to have the easiest time working in Python.
Intel Euclid Development Team