Naturally, the world is never uniform in its progress, a welcome reality that reflects the diversity in its denizens. It therefore should be no surprise that any two given individuals (even within the same city) will have varying degrees of proficiency in a mutual second language (with the assumption that they already speak the national language fluently). If, for example, two Canadians both happened to also study Mandarin, where person X is reasonably proficient, but person Y is just a beginner, it would be unfair of X to speak long strings of Mandarin to Y, since Y has no facility to parse or perceive X's utterances. Actually, unless there was a reason to speak in Mandarin (like, say, a Mandarin tutoring session), I'm usually of the opinion that one should stick to English (in the context of living in Canada). This sentiment is probably best captured by Emily Post in her book Etiquette (1920):
"Never interlard your conversation with foreign words or phrases when you can possibly translate them into English; and the occasions when our mother tongue will not serve are extremely rare."The reasoning behind it, at least from how I interpret it, is that when you're in a speech community in which everybody speaks English, but not everybody speaks Mandarin, any Mandarin utterances would alienate non-Mandarin speakers. And, after all, the goal of etiquette is largely the art of making all parties present to feel well at ease.
However, this rule is only solid in a non-lingual situation. Consider, for example, my personal situation in the German Club on campus. I'm fairly confident in my opinion that of all parties present, I probably have the weakest handle on the German language. And yet, it would be rather ridiculous to demand an English-only environment in a club that purportedly is for the German language [and culture].
So how would a socially responsible individual resolve such a conflict? I think it can be done in a combination of the following ways:
1. Speak slowly in the L2
That is, in a country that speaks X, and a club/society that speaks Y, language Y should be spoken slowly and clearly for the weaker students of Y. Why? Because those who are just beginning to study language Y, or who aren't as strong in Y will have more trouble parsing all the sounds in Y. Also, the speaker should be willing to repeat him/herself.
2. Offer translations in the L1
Which is to say, if there's an item in the L2 that's particularly idiomatic or culturally entrenched, it would do well to have an explanation handy. Again, weaker speakers of the second language will obviously have more trouble following the conversation or dialogue.
3. Be inviting
Suddenly switching into another language, even in a context-appropriate situation as a language club, can be rather isolating and alienating. It's important to make the weaker speakers feel more welcome. Afterall, we were all beginners at one point. Usually, I'd only start with insert words or phrases, to get the kids going (as in the Japanese culture club, where my language skills are markedly stronger than my German).
Does this mean that everybody involved in language learning has to be a specialist in SLA? No, not exactly. But I think that anybody who has been in a beginner SLA situation can relate to the feeling of being left out. Obviously there are two different responses that can be had -- some individuals react by vowing to learn the language better so that they can become the alienators. And of course, some clubs want to prune out the weaker speakers so that they can enjoy pure L2 conversation without having to worry about L1 explanations.
Unfortunately, that usually means reduced numbers in the club/society, and it also means a severely reduced scope of perspective. 'Sides, the more the merrier, right? Exclusivist clubs tend to die out, especially without an extraordinary amount of funding. So, the choice is yours! As for me, we'll see if I'm able to stick around in the German Club. Nobody's been making me feel particularly isolated, but on the other hand, the language barrier is clearly something to be overcome.