Make sure kernelspec language is good for every kernel

[thomasjm]

This value is used as the interpreter info name in the runner, i.e. as the key of a map of the running interpreters. Thus,

• Every kernelspec should have one
• It should be unique to the kernel / kernel version. I.e. if we have the ability to install multiple kernels of a given language, they should have different kernelspec languages.