> Well, but that's not too different from the use of Italics in English orthography, which also includes sound effects and foreign words.

True. But using italics with foreign words is not a de facto English language rule.

Please note that Japanese words themselves can also be written in katakana, sometimes as a sort of emphasis. Other reasons can be it being a brand/name, or because it is easier on the eyes, or because the kanji are too difficult. So it's not like every word written in katakana is a foreign one.

That said, most words written in katakana are probably foreign.

This is a nuance to Japanese I did not know. Thank you for educating me. :)