I live in a predominantly Catholic culture in Mexico, but in a bordertown, so it has America's hardcore hypocrtical religulous population alongside a pretty indifferent atheist approach to life.
I don't particularly feel it's anyone's business that I'm a Buddhist, it's not about me. I always have an answer ready to go though if asked about Buddhism as my friends know I'm Buddhist because they've explicitly asked. My answer to go is like Keith's. I lay out the 4 noble truths in simple and daily language. Nothing fancy, nothing exotic, just what we can all resonate with.
I don't think there is anything right or wrong in expressing one's faith, it's a matter of choice in how you want to face your day. If you are ok with religious attention, you present your religiosity to your peers, and that can be a wonderful and inspiring quality or can be preachy and in your face, depending on the day.
I'm not all that skilled with the topic when it comes to monotheistic paradigms so I tend to steer away from the religious emphasis of any given situation. For example a good friend of mine has a very Catholic mother, whom I care a great deal about. When things happen, she often tells her daughter it's god punishing them because they didn't do the *insert right thing to do here*. I don't argue the bit about god, instead say, Ok, even so, God punishing me does not need to prevent us from doing the right thing now. The facts are. This or that happened. The facts are, to make this or that right, we have to do this or that action to amend for the mistake. Even she can't argue with that. I'm not saying God isn't punishing us, I had an aunt who used to spout some hindu karmic stuff about people getting their dues, haha, but really, it doesn't matter which power that be did what, the most important thing is to see the situation for what it really is and not how it feels through the lens of fearful emotions.
This means normally I don't care for the religious attention. Too often it tends to be a divisive topic rather than an inclusive one when dealing with monotheisms, not only but it pushes people's deepest buttons if they're serious believers, and I live by the creed: Primum non nocere. Cura te ipsum. First do no harm. Cure your self instead.
The only time I discuss religion is when I can see clearly that addressing it in conversation directly benefits the person listening, in particular it benefits them in healing some part of themselves. I always ask myself how anyone knowing something about me benefits them. If it does not and it only benefits me, I've been practicing refraining when I speak... Most people give you clear cues if they are really curious about dharma. They usually let you know that they want to know or learn about Buddhism and ask you direct questions about it.
It's ironic that I have this view point and I happened to speak with a friend about dharma a few months ago. She has since asked three times, and the first two times I simply said what seemed fit for the circumstances, mostly I'm really a beginner, not qualified to explain, but it is a path in helping those who need help. I help because I know what it feels like to be alone and in much pain and need the help.
The third time she asked me and asked me to do an interview on her TV show on a local tv channel in town. I shared with her that I am Buddhist because I practice dharma, and not because of any ritual or church. She wanted to know about esoteric practices and asked about astrology and tarot. I had mentioned that I had studied these forms before coming to Buddhism and had explored many forms of truth. However, this is the truth I have experienced in all that I studied. Esoteric or mundane forms of understanding life and systems of mind like astrology and tarot have no more power or magic than my morning cereal.
Truly it is the mind that guides itself, not something divine outside the being right now. Whether there is or isn't a divine being outside my house in the sky may well be, I have no way to say either way, but I have lived this life and have suffered this way thus far. And I don't want to anymore, but I realize I've come to this place because of the ways I've chosen to see things in time, and now my perception is what I see through. But what I see isn't necessarily the truth, what seems isn't necessarily what is.
Immediately, regarding interviewing, I didn't feel comfortable speaking for "the dharma" to local Mexico as a complete noob to Buddhism and practice. I wouldn't say 2 years of having taken refuge qualifies me as much more than noob.
I thought it over and realized, I could speak about something I knew about. Kindness. The four noble truths. Taking responsibility for ourselves by first of all breathing and being present. So on the third time she asked me to interview on her show, I said, ok.
We met for the show, I talked about something simple. The warm tenderness that gets elicited from daily experiences, and how we can see in them the 4 noble truths. There is suffering, but we are intelligent capable beings who can help alleviate ourselves by being present, by keeping warmth in ourselves, and not becoming rigid to the life we encounter. Compassion comes from knowing suffering. Without suffering's existence, there would be no need to have compassion, everyone would be fine. But the whole world hurts, big and small. Everyone is subject to hope and fear, therefore everyone goes between pleasure and pain. We hate losing what we love, we hate things changing from comfortable forms to uncomfortable ones, we don't much like getting what we want to avoid, these things are hard to tackle every day.
I spoke of the things I usually speak of, which aren't religious per se. They're simply human. I told her the path I practice advocates guardianship of those with no protector, caring for those in need, and helping the weakest become self sufficient. In Mexico we could do this by making sure that those who don't have are taught to be able to provide for themselves, even if we started small with just being present in the moment, we can reach it. I asked the audience at home to just try taking 10 deep breaths without thinking, without the inner voice talking. I said, you count and you see. If you can get to 10 you are doing far better than I when I first started breathing normally.
I talked about appreciating life in every stage, and how even though some days are more bitter than others, this is perfectly human, what is the alternative? There is no alternative to the present but the one we're in. So, we have the ability to truly transform in real time, to stop being what is damaging, in every moment that we are, we can know who we are in what we do and how we see ourselves and the world around us.
I showed a mala, a singing bowl, a prayer wheel. I spoke of how these are tools of prayer, but not prayer to a god necessarily, prayer in the form of concentrating intention, goodness and compassion into the moments of each day. Reflecting on this very moment, the beginning and the end of your breath is the core of the beginning of life and the ending of death. We use what we encounter in our lives, ordinary as they may be as opportunities to build a momentum kindness, sharing, and truly joy. Because joy that is shared is the elixir of life.
She asked me if Buddhists believe in god. I said, it's not that Buddhists affirm or deny the presence of a god per se, whether there is or isn't is simply not relevant to the you-and-me daily life. Can god appear right now and take this arthritis from me please? Not really, I need a doctor. So I take medicine. Buddhism is like medicine for being present and clear.
There is no real conflict with following your culture's customs, and many of my Catholic friends here are more by culture than any real belief in God, there is a rich heritage tied into the local celebrations of food and love and life. You cannot go somewhere and tell people they need to change their way of life. That's very harmful. You must respect the environment of culture. Catholicism in Mexico is a deep part of the people's way of relating to daily life.
My belief on this is that you can still believe in your culture's ways and have clarity of mind. Dharma is a method to find the cessation of your own suffering, the universal language of wisdom that speaks through cultural appearance. Really all it asks you to do is stop and relax long enough to know what this moment is made of. Who knows what a clear and tranquil mind can do. No matter what religion you're from. There are simple or complex techniques, depending on the mind doing the thinking. Dharma is truth that takes the shape of its practitioner's actions. We need our humanity. It is what carries this truth across language and the barriers of body.
What I think is that being a Buddhist in a Christian/Catholic society does not necessarily mean there has to be a conflict. People like using logic, wisdom, why are elders so respected in so many societies? Because there is a strong presence of respecting wisdom built within the fabric of the culture's aesthetic. Except America. It throws away its old like trash. Puts them in homes to die without the warmth of their families, because people have lives that are too busy to take care of those who took care of them.
Being a Buddhist in a Christian society involves a lot of trials of "self" and not being the center of my own world, because I have to deeply respect the matrix my friends come from the way they respect mine. And I still respect those who don't respect mine, because well, that's on me, not them.
Thank you for such an inspiring question!

D. Ogyen