This question came in on Quora just when I had spent three days with two of the greatest polyglots alive: Professor Alexander Arguelles and Richard Simcott. Both know more than 30 languages. We had also organized a polyglot dinner where people could speak 10 languages on average. All in all, I was in a unique position to answer this question and had a lot of fun doing so.
When being introduced at a polyglot dinner, everyone would speak English. Then, there was often a trial period where people would repeatedly switch languages in order to assess each other’s levels. This would happen without agreement or warning, just answer in a different language than the one you were speaking or add something like “Natürlich sprichst du auch Deutsch?” (of course you also speak German?).
Dinner with polyglots
The language that was finally chosen was often not people’s native language. For example, even though Professor Arguelles and Richard Simcott are both native English speakers, they spoke very little English with each other. Instead, people will communicate in their best “non-boring” language. “Boring” languages being ones that they already speak every day and have reached maximum fluency in. The idea is that you should learn something from speaking this language but without obstructing the flow of conversation. Adjusting for everyone’s comfort of course, because it would feel too unequal to communicate in a language that one person is completely fluent in and the other still regularly searches for words. So when Richard and Alexander recorded a short video where they talk about language-learning (forthcoming), they chose to do so in German, which they both speak perfectly, so as not to give even a hint of one-upmanship. At other times they also spoke a significant amount of Italian, Spanish, French and Russian to each other, and less often 8 or so other languages.
It’s another thing if the purpose is to practice. When polyglots meet, it’s often also an occasion to try out rarer languages that they don’t often get to speak where they live, for example Modern Greek, Modern Hebrew, Welsh, Esperanto and so on. At the polyglot dinner we hosted, with 8 people attending, more than 20 languages were spoken (in different pairings) over the course of two hours and at one point the entire table was learning phrases in Indonesian. The Polish-American-German group at the table next to us actually came over and asked where we’re from, because they heard so many different languages. A great occasion for Richard to flabbergast them some more by explaining in fluent Polish, which we hadn’t spoken yet 😉
Whichever languages you choose, being a polyglot and meeting another polyglot is great fun.