What is necessary for multilingual speakers to engage in a bilingual mode…

Concept — In bilingualism research a speaker's language mode is the relative activation of the languages they know at a given moment, and it runs along a continuum. At the monolingual end one language is active while the others are held back; at the bilingual end two or more stay active together, so a speaker may draw words, phrases or whole clauses from either. Where a speaker sits on that continuum is set by the situation of the exchange — chiefly the interlocutor and the norms of the setting — not by how well the languages are known.

Application — The stem asks what has to be in place before multilingual speakers actually operate at the bilingual end of that continuum in a live conversation. Bilingual mode is interactional: it takes participants who share the languages and who treat drawing on both as fitting for that exchange. Speakers reach that state through a shared, usually unspoken, understanding that using both languages is appropriate here — a mutually established agreement on appropriateness. Remove that shared understanding and the same speakers stay in monolingual mode however fluent they are.

Contrast — Checking the other candidates by value:

• The absence of any formality: formality is a register dimension that shifts inside a single language as well as across languages. Formal settings often pull speakers toward monolingual mode, but bilingual mode also occurs in formal exchanges where mixing is the accepted norm, so an absence of formality is neither required nor enough on its own.

• An automatic response due to bilingual competence: competence is what the speaker carries into every situation. The same competent multilingual speaks monolingually to a listener who does not share the second language, which shows the mode is selected by the situation rather than triggered automatically by the knowledge.

• A strict alternation after each sentence: documented code-switching in bilingual mode operates on units of many sizes — a single borrowed word, a phrase, or a full clause — and is not governed by a fixed sentence-by-sentence rhythm.

Result — Multilingual speakers enter bilingual mode when the participants share a mutually established agreement that using both languages is appropriate in that conversation.