What is the generic term for a person who knows and uses multiple languages?

Concept: English builds its vocabulary of language ability from a Greek numeral prefix joined to glotta / glossa, "tongue": mono- means one, di- means two, and poly- means many. The ending -glot forms a noun naming a PERSON who commands that many languages, while the ending -glossia forms an abstract noun naming a language SITUATION in a speech community rather than an individual.

Application: The stem asks for a generic term — one label for the individual, not a description of a job — for someone who knows and uses more than one language. The sense "multiple" selects the prefix poly- ("many"), and the sense "a person" selects the person-forming ending -glot. Joining the two gives polyglot, the standard noun for an individual who commands and uses several languages.

Contrast: Each of the remaining words breaks one of those two requirements:

• Interpreter names an occupation, defined by the act of rendering speech from one language into another as it is delivered. It describes what the person does for a living, not how many languages the person holds, so it is not a generic label for multilingual ability.

• Monoglot carries the same person-forming ending -glot, but its prefix mono- fixes the count at one language, the opposite of "multiple".

• Diglossia ends in -glossia, so it names a community-level condition in which two varieties of a language are reserved for separate formal and informal uses. It labels a linguistic situation, not an individual.

Cross-check: Change the prefix and the meaning tracks predictably — monoglot (one language), diglot (two), polyglot (many) — while no -glossia noun ever denotes an individual. Polyglot is the form that satisfies both requirements at once, "a person" and "many languages", so it is the generic term the stem asks for.