Learn English – When did “text” come to be defined as something other than words

The Patrician, Lord Vetinari:
'I shall deal with the matter momentarily,' he said. It was a good word. It always made people hesitate. They were never quite sure whether he meant he'd deal with it now, or just deal with it briefly.
And no-one ever dared ask.

Guards! Guards! - Sir Terry Pratchett

The short answer is that cursing is much closer to being universal than the way you portray it in your question, and English is not by any means unique in its handling of taboo. The subject matter may differ (though a huge number of different cultures seem to incorporate a few fundamental taboo concepts over and over again), but the function and ubiquity of curse words occurs in every society, including Japan. From Jay (2009):

For example, a Japanese Touretter is likely to yell ancestral allusions (shit grandma!) because irreverent references to ancestors are extremely taboo in Japan.

I don't know enough Japanese to actually weigh in with my own opinion, but I understand that the idea that Japanese doesn't have "bad words" is a myth. Here is a group a reader-submitted anecdotes, many of which touch on Japanese curse words. While claims of "this language has no curse words/only one curse word" are very secondhand, I assume most of the specific references to Japanese curse words are not made up (someone with better knowledge of Japanese can weigh in there).

Cursing itself is such a fundamental act, many consider it to be extra-linguistic; from Fitch (2010):

It is quite common for aphasic patients to have severely disrupted spontaneous speech, but to still laugh, cry, and curse normally. The fact that (learned) words involved in cursing can remain unimpaired after damage to the rest of the lexicon, and tend to segregate with innate calls, is quite interesting. It suggests that the linguistic system has in some sense partially 'colonized' the older and more basic brain circuitry involved in emotional expression (Myers, 1976).

Swear words seem to be some of the few words that can operate in ways that violate fundamental rules of syntax, as well. For example, we can say "what the fuck" or "what the hell", which, grammatically, makes no sense. Dong (2007) illustrates in much more detail (and yes, this was published under a pseudonym) that fuck violates many syntactic rules in unique ways.

The function of cursing might well be to alleviate stress or pain.

Taboo words evolve in a number of ways: over time, a specific word can lose its strength, but also, shifts in taboos and the severity of taboos can have a huge impact on the severity of a curse word. For example, bastard was a much stronger curse centuries ago than it is today. On the other hand, the recent development of e.g. retard into a curse word is reflective of the shifting attitudes towards the mentally challenged (i.e. how belittling/dehumanizing them has become taboo).

So, regarding your question of whether English was always this way: I think most linguists, neuroscientists, and psychologists would say that language seems to have always been this way. I'm not sure what you mean about English curse words being segregated into a fixed group. There are a lot of ambiguities surrounding English curse words.

• the list of words evolves and changes over time.
• some words can be said on TV, some can't; some can be said on TV in the UK but not in the US.
• some words are used almost exclusively as curse words ("asshole" rarely refers to the sphincter), while others are commonly used propositionally as well as emotionally (e.g. "shit").
• some words can be used publicly with certain meanings (e.g. you can say pussy on TV as long as you are referring to a cat, but not if it is a sexual reference), so it is still contextually dependent. People can always say damn on television, but if they say "God damn", the word God will be bleeped/blanked out (and not damn): again, very contextual.
• And so on.