General:
* Linguist
* Language expert
* Language professional
* Language practitioner
* Language consultant
Specific areas:
* Translator (for translating languages)
* Interpreter (for interpreting spoken languages)
* Lexicographer (for creating dictionaries)
* Phonetician (for studying the sounds of language)
* Sociolinguist (for studying the social aspects of language)
* Psycholinguist (for studying the psychological aspects of language)
* Computational linguist (for using computers to study and process language)
* Language teacher (for teaching languages)
* Language tutor (for providing individual language instruction)
Other options:
* Language analyst
* Language researcher
* Language developer
* Language engineer
The best synonym will depend on the specific situation. For example, if you are looking for a synonym for someone who translates languages, then "translator" would be the most appropriate.
It's also important to consider the level of formality required. "Language specialist" is a relatively formal term, while "language expert" or "language professional" are more casual.