Don't assume that no-one else around you will understand your native tongue. Sure, it might be tempting to say something snarky about people you see, but you never know who might be able to understand you.
If you speak Navajo you might be OK, bit there are often surprising people around. You might even run into an Asian-looking man in Portugal that understands Swedish and helps you find your train... Or people in the USA that understand Afrikaans.
All this was prompted by a South African family that sat behind me on a recent flight from Seattle to Oakland: They used Afrikaans to make loud rude comments about other people, discuss their flatulence and generally behave boorishly. If you want to skinder* about people, do it under your voice so no-one else can hear... Or better yet, just don't do it :)