Living not at home

How do two multilingual people choose which language to use? If they both speak the same pair of languages, how do they choose which one to hold their conversation in? What are the social conventions? The choice of language may be based on the dominant language of the place: they use whichever language most other people around them are speaking.  It may be based on facility: their levels of proficiency in the two languages will be different. This means that one of them will have to speak in a language at which they are less adept than the other person. They use the language in which the gap between their respective levels is the smallest.  It may be based on precedence: they’ve always used only one of their two available languages. Their relationship exists in one language, but not in the other.  I live in Portugal and most of the people I speak with have Portuguese as a first language. When I begin talking, it’s obvious I’m not fluent in Portuguese: my accent is a bit of...