Actually, this problem does not only happen in a conference: I think there is two slightly distinct situations in which you can be surrounded by people speaking in a language that is foreign for you. The first is as you described: conferences and other short-term events. The second is (if studying abroad) coming to a language-homogeneous (working/research) group.
In both, unfortunately, the only way "out" is to be at least slightly assertive. It is not the most pleasant, but it's natural for the (larger) groups that speak the same (mother) language to revert to it naturally in their conversations unless they are reminded to speak in English.
For the conferences, I would say, it might even be easier of the two:
- in my experience, the first social event is some kind of a "mixer" where there is no fixed seating arrangements for the whole evening, so it's easier to identify the people open for conversations
- also, approaching small groups of 1-2-3 people on poster sessions and lunch breaks can be a way to meet people who want to communicate in English
- the "seating" dinner is usually a few days into the conference, so by then I just try and locate the people I talked to during the last few days
For the more "permanent" group, e.g. your team, it is a bit harder. They are an established group already most likely, and you want to get in as well as make them change their standard communication language. For that, you have to get noticed.
- there's always a problem of finding a balance between being present, noticed, and too aggressive
- establishing your presence in the group by attending team coffees, greeting people in their offices as you come to work, and similar small gestures should get you well started
- on the longer "group outings" (my team used to do that - going out together a few times a month), it's usually easier to start a conversation with just one person, and hopefully other people will join in.
- finally, it's okay to sometimes gently remind them that you'd like to participate in general conversation as well, as long as you don't come across as rude or judgmental. Nobody likes to be shamed in public.
But, the bottom line is, staying politely quiet won't get you far. You have to get noticed, you have to be interesting to others, and you need to make them want to speak to you.
I know that some people are not as socially comfortable as others, and I know that all that can be hard, demanding, sometimes exhausting, and you don't always feel like it. Unfortunately, I don't know or see any other solution.
So, the best "tactic" I came up with is: when you know you will be in those kind of situations, actively mentally prepare. It's easier if you know you have to do it, and it gets a bit easier every time. You take a deep breath, you dive in, and try your best to be a social butterfly. When you know you are attending a conference or coming to a new working environment, be prepared to put more effort in getting noticed for the duration of the conference/next few weeks.