A philologist I know constructed a sentence like this:
The choice of language was determined by the need of integration of the module in the system, and by the fact that [language] has a large number of open libraries that make it possible to work with [...] technologies, what is an advantage in the [...] processing.
She's sure that the usage of "what is" is the sole correct option in this situation, while I'm sure that the correct way would be to use "which is".
Neither this philologist nor I am a native English speaker.
What is correct in this situation? And what is the rule for this?
Edit: Made the sentence more specific. I cannot publish it in full, so some parts were omitted.