Which coordinating conjunction is used for contrast with negation?

For example, the sentence "Er spricht nicht nur Deutsch, sondern auch Französisch."

The coordinating conjunction that is used to show a contrasting idea or correction is 'sondern', which means 'but/on the contrary'. 'Sondern' is used specifically after a negation to introduce an alternative or a contrasting statement, implying a correction or refinement of what was negated. It's similar to "but rather" or "but instead" in English.