C++ and Python nodes in the same launch file?

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C++ and Python nodes in the same launch file? Is this possible as long as I use proper syntax?

yes.

Or will I have to separate into one launch file per language?

no.

In fact, there is no assumption about any language or binary format. As long as what you try to start is:

1. located in a ROS package (it is possible to work around this, but in general this is going to be the easiest to get working)
2. is executable by the platform/OS you're running roslaunch on

It isn't even required for executables to be ROS nodes (non-ROS programs will receive a couple of extra command line arguments, but if they can deal with those, they can be started. Otherwise, see: #q272267).