Okay, basis a quick check, I think I do have some tentative answers, listed below simply in order of how I recall what I read through just now:
1. Yep, IUPAC does set the rules for all names of all chemical compounds.
2. It sets rules for naming of all compounds, not just organic but inorganic as well. And elements as well. And much else besides, like units and stuff. They’ve got separate manuals for organic compounds, and inorganic compounds, and other stuff as well, which they denote by color: Red Book, Blue Book, Green Book, etc.
3. As far as I could gather, IUPAC names new elements or chemicals itself, or at least ratifies them. That is, it actually involves itself with every name, of every new chemical compound that is discovered or made, is what I gather.
4. That said, there’s still a great deal of divergence about names of chemical compounds, for a number of reasons:
(i) IUPAC was formed only about 100 years ago, so names from before then sometimes get carried forward simply through inertia. Informal names, that vary sometimes by country and language.
(ii) Commercial/brand names become popular sometimes. That too, absolutely.
(iii) IUPAC isn’t the only body that sets rules for naming, and names compounds. Even as far as chemistry itself, there’s many other bodies. Like CAS, and IUBMB, and InChiI, and SMILES. They aren’t rival bodies, all of them act in agreement with IUPAC, and in fact at least one of them was set up by IUPAC itself, it’s just that they assign names to fit “mechanical retrieval”, to help with easy indexing, and basis the requirements of such other specific purposes, --- given that IUPAC names can be huge and unwieldy.
(iv) Outside of pure chemistry, there’s other bodies that set the rules, and the names, for chemical compounds --- like medicine, public health, biochemistry, …etc, etc. That’s because they find IUPAC names too long and unwieldy, and unnecessarily detailed, so they set their own names basis their own rules. And in some fields, like Medicine for instance, different countries can have their own different bodies, whose rules and names again may differ from one another sometimes. So, the same compound can have different names, as assigned by and basis the rules of these different bodies.
(v) One added layer of complexity is when all of these are translated into other languages. Over and above the differences already discussed. (Don’t know if that’s the case with our particular Japanese word, upthread: but if you simply compare a Japanese name of a chemical compound with an English name, then, unless someone that knows what they’re about, makes fully sure to compare like with like, then you may end up with very different names, as discussed above. And add to that differences on account purely of language, script and pronunciation structure specific to individual languages.)
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ETA: Yep, that's about it. Just:
www.dictionary.com
