That sentence sure looks odd to me, michael13. If I wanted to express that idea, I'd forget "in" and "on" and use another structure: He is a linguist who studies English.Thank you Cyber.
But a professor is usually a teacher at university, while a linguist could be or not. Do you think
eg He is a linguist on/in English.
is not English?
No, "he is a linguist in (or on) English" won't do.If an English linguist might be interpreted as a linguist from England or as a linguist specializing in English, what is the most succinct way to refer to such a person? How should I phrase/improve my sentence?
e.g. He is a linguist on/in English.
Your initial comment there reminds me of the old joke about the guy who could speak twelve languages fluently, but had nothing intelligent to say in any of them." ... linguist" implies a level of skill at languages generally or at several different ones".
In my (alas!) long career in the linguistics arena, I've encountered superb linguists who couldn't express themselves decently in any foreign language; I've also met fluent speakers of different foreign tongues who didn't know the basics of linguistics.
I think it is very English (British?) to use the term "linguist" to refer to a polyglot.
In Continental Europe we tend to use the noun "linguist" — in the old days many would use the term "linguistician" — as derived from linguistics, not from language(s).