Tbh, this doesn't only happen in malaysia, but pretty much everywhere else in the world. I have been on the receiving end and also, the one who's been doing it unknowingly.

1 thing you have to understand is that people are used to conversing in their own mother tongue. I had housemates from Penang and despite being able to speak perfectly in english, canto and mandarin, they speak hokkien to each other most of the time and they can just switch to hokkien in the middle of a conversation with me and make me feel invisible. So i got sick of it and just don't mix with them much.. until all of  sudden, like a scene from the mean girls movie, they just totally tak suka me and tells me i'm anti-social. When i tried explaining why I do what I do, their only response is "then learn hokkien lah. whats so hard?" ... So you see, chinese can do the same crap to chinese.

Now, I have this indo-chinese colleague who totally can't speak chinese, zilch, and everyone in the group which I have lunch with, are singaporean chinese. Every time we go out for lunch, they'll be speaking in chinese and the indo-chinese guy will sit there, feeling lost. Having been on the receiving end before, I will constantly try to switch the conversation back to english. Sometimes it works, sometimes, it doesn't. Sometimes, I myself, will keep speaking in chinese until i caught myself doing it, then i will switch back to english and try to explain to that colleague what the conversation is about.

I work with clients from different countries and they all behave the same way. When a group of french are together, they'll just start speaking french and forget about your existence. Same goes to indians, germans, etc. Only exception is unless you're a guest in their country. Then, they will speak in a language that you understand. Common courtesy.

It's just habit; not a race thing. Don't play the race card.

Now, what can you do, you ask? You can either:

1) Casually butt in whenever you feel like you want to be a part of the conversation and jokingly say "aiya, cakap apa? saya mau faham" or things like that
2) Suck it up and don't complain but continue to harbor bad feelings
3) Report to management and make a big hooha
4) Go learn the language

The 1st one works like a charm for me. Not sure which one you'd like to try but good luck