The interpreter’s presence makes a conversation possible while adding another person to its privacy.

Communication becomes possible and the room now has a third person in it. That is a genuine trade: the conversation can happen, and its privacy has changed shape, and both parties have to trust someone whose presence they may not have chosen. Access sometimes costs intimacy.

Handle it by acknowledging it rather than pretending the interpreter is furniture. Address the person you are speaking with, not the interpreter. And be aware, in any situation where someone needs assistance to take part, that they may be giving up privacy to do so, which is worth respecting with discretion rather than curiosity.