General:
* Linguist: This is the most common and general term. Linguists study language in all its aspects, including its structure, sound system, meaning, and use. They can specialize in different areas, such as phonetics, syntax, semantics, or sociolinguistics.
* Polyglot: This term refers to someone who knows and is fluent in multiple languages.
Specific Areas:
* Philologist: This term is often used for someone who studies the history and development of language, especially ancient languages.
* Lexicographer: An expert in lexicography, the process of compiling dictionaries.
* Translator: An expert in translating text from one language to another.
* Interpreter: An expert in interpreting spoken language from one language to another.
* Language Teacher: An expert in teaching languages to others.
The best term to use will depend on the specific context and the expert's area of expertise.