For those who don’t know, here in American English, “talking smack” or “talking shit” means making insulting or critical comments to someone, especially an opponent. I suppose the British equivalent would be “taking the piss” or “taking the Mickey.”
How do you express this concept in your language. I’m specifically asking for colloquialisms, and not the dictionary words for “insult” or “disrespect” or whatever.
“Taking the maple syrup out of the beaver”.
No, actually it’s the same up here as the US.
I’d argue these are related but not synonymous with talking shit (as I understand it as a brit who’s been the states 20+ years). The key point of taking the piss is you are mocking someone, the key point of talking shit is you are demeaning or criticizing them. There is some overlap (there is usually some demeaning involved in taking the piss and some mockery involved in talking shit) but they are not the same.
I can’t think of an exact British equivalent of talking shit. But maybe I’ve been in the states too long 
Slagging off?
I think you’ve nailed that one, ‘slagging off’ sounds perfect as an equivalent.
While I agree that it is the same up here as in the US, I do have to compliment you on “Taking the maple syrup out of the beaver.” Well done!
Gotta agree with @griffin1977 that they’re not equivalent.
In South African English, “talking shit” is exactly the same as it is in America, but also can carry the meaning of just talking nonsense, not necessarily being insulting. Same with the literal Afrikaans translations “Jy praat kak” or “jy praat strond”, which more carry often that nonsense meaning not the insult one (other than “You’re insulting my intelligence”) , but can.
“Don’t come the raw prawn with me!”
I’ve never heard anyone actually use this phrase, but it’s supposedly authentic.
Are you training Ai?
Talking shit about someone is disparaging them and probably for good reason.
Taking the piss is mocking them. You can ‘take the piss’ from a best mate, or an enemy, but more likely a mate. You wouldn’t talk shit about a mate. Well, you might, but he won’t be your mate much longer/Australian
In Hebrew, I think the comparable phrase is “לרדת על” - “to go down on” someone.
Obviously, this does not have the sexual connotations it has in English.
Milder versions (UK) of “slagging off” would be “having a go” at someone, or "running down’ someone. They’re all entirely hostile, whereas “taking the piss/Mickey” can be more ambiguously affectionate (?) teasing, or “winding someone up”.
Sending an apprentice for a left-handed spanner would be taking the proverbial, bawling them out over some triviality would be slagging them off.
Yes, it is authentic but it doesn’t mean the same thing. It is telling someone that you think that they are trying to trick you, “put one over on you.”
More in keeping with talking smack in Australian speak is sledging I would imagine.
I think ‘gobbing off’ would possibly also fit (and the possibly more polite version ‘mouthing off’). It is a term that just means excessive talking, but with a shade of meaning toward saying stuff you shouldn’t say; often means ‘boasting’, but not exclusively.
Whereas excessive talking without any particular emotional colouring would be “rabbiting on” (rabbit[+pork]=talk).
“chirping” - Canada, related to sports trash talk level.
I don’t disagree, but these days I think to diss someone is another UK English possibility.
j
US in origin, apparently.
You can always do it through the medium of dance:
As a Brit who has lived in the US for much of my life, I have to say I haven’t really heard the “talking shit” ideom much at all.
Is it perhaps a regional thing? Does it imply saying it to someone’s face, or talking about them to a third party?
I’d agree that ‘slagging off’ seems a close UK relative, though that usually means to a third party rather than face to face.
In UK English, “talking shit” would usually mean you are spouting nonsense or rubbish, I think.