There is quite a bit of confusion online about these two terms.
1: a person accomplished in languages; especially: one who speaks several languages
2: a person who specializes in linguistics
We can see from the start that the way the word linguist is defined and used in modern English is ambiguous.
The word is rather often used in military job ads as “army linguist”. The GoArmy website defines linguists’ responsibilities as follows: “translate highly classified documents and information for military troops and allied forces” 1)
At the same time, the image shown on that page is people talking and not translating documents.
The army interpreter/translator is defined as a separate category i.e. “primarily responsible for conducting interpretation and preparing translations between English and a foreign language.”2)
In an online forum army interpreters/translators (09L) are defined as “native speakers that primarily just translate” and army linguists (35P) are defined as “intel Soldiers who collect information and use their language skills to sort through it.” 3)
We can see that this definition of linguist is as remote as it may be from what conference simultaneous and consecutive interpreters do.
The first definition of the word linguist from the Merriam-Webster Dictionary is better described by the word “bilingual” or “multilingual person” i.e. someone who speaks two or more languages. Polyglot is not exactly a technical term.
The problem here is that “accomplished” is not a technical term either: there are various degrees of bilingualism and multilingualism for various purposes: a child who can help his Spanish speaking parents understand his English speaking teacher very well is certainly an accomplished bilingual for this task and his age, but he is not a linguist.
We suggest that the first definition of the word “linguist” from the Merriam-Webster Dictionary should not be used at all, and certainly not in the meaning of “interpreter” or “translator”. The first definition may also be more American English usage.
Leaving aside the difference between interpreters and translators (oral speech versus written text), the interpreter does not have to be a linguist as per the second definition of the Merriam-Webster Dictionary.
Of course, knowledge how language works is useful but it does not make an interpreter. Linguists study differences between languages not the process of professional translation or interpretation.
What is required to be an interpreter is dedicated professional postgraduate training in language interpretation – formal education and a degree in conference (simultaneous and consecutive) interpretation.
Therefore, linguist should not be used to denote a language interpreter.