Martin, for someone who isn't even a native speaker of the language, you sure are a drag about it.[1] Languages evolve, new words appear, often through silly slang like this. I find "holla" to be an atrocity myself, but claiming it's not a word is just plain silly at this point.
Why do you not consider it to be a word? Because it's a rewriting of an old word and not added to most dictionaries [yet]? All words were invented by someone at some point; some caught on, some didn't, but they're words nevertheless. This one's certainly caught on, even if it's mainly in some circles. Is "OK" a word? It's slang, after all. Originally an acronym for "Oll Korrect", if my memory serves me right, so it's quite comparable with holla.
How long must something live on before we get to call it a word?
We could probably go on forever about this, I suspect! Oh, and I really, really did not mean to sound as patronizing as I did here, but rewriting the post seemed like more work than it's worth.
Now where's my fropping whamostrix glumpadill tutt? [1]
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[1]: it seems Opera doesn't handle the insert-smiley buttons. O well, there could've been an exciting winkyface[2] up there)
[2]: not a real word! Or if it is, it's probably actually an insult. Whoops.