I'll just leave this here for cultural context.
This is nothing against translation, but rather a bit of a nitpick regarding mangaka's own depiction of Thailand. I understand that the point was used to highlight difference in cultural norms and thus the need for the characters to adapt to a new environment, but... you can definitely throw toilet paper—especially dissolvable ones—into most toilets in Thailand, including ones at the international airport.
There are some public toilets where you will find signs telling you not throw "trash" because unfortunately people were not the most discipline here in Thailand and used to throw all sorts of things into the public toilets, including things like plastic wrappings, cardboards, menstrual/sanitary pads etc. And Thai's managing bodies in some places—being as incompetent as they usually are—just decided to blanket "ban" everything that's not bodily excretions including toilet paper because it's shorter to say, "don't throw ANY trash into the toilets," than it is to say, "don't throw anything but used toilet paper in the toilets." You don't usually see these kinds of signs anymore though, and so I hate that this misconception about toilet etiquette in Thailand is still around after more than a decade or two when some dimwit thought the blanket policy and those misleading signs were a good idea.
So for the love of hygiene, if you do visit Thailand and there is no sign EXPLICITEDLY telling you not to throw toilet paper into the toilet, please throw your used toilet paper into the toilet and not the trash bin.