i see.
just stating my quick opinion.
I think mixing languages is normal and that's what happened naturally if the group is taught with more than 2 languages.
in history, people has been mixing languages officially.
example, in BM, garpu and almari is borrowed from portuguese (garfo and armario), mustahil from arab, etc
slang will be different from place to place because language do "evolve". look at British and American english, even the spelling and word usage evolve.
the newest language in the world, afrikaan, was mixture of dutch and south african language.
but i've seen some people making big fuse of mixing language, like they don't like people who mix english with bm, chinese from mainland china don't like those malaysian who speak chinese in malaysian version, chinese malaysian highlighting that mandarin, hokkien, and cantonese in malaysia is not pure because they borrow vocab from BM, english speakers in malaysia not using pure english, etc etc.
Malaysians especially peninsular people like to refer money as "lui" instead of "chin".