Thanks for the clarification! That said, couldn't it just be that the First Hero was Japanese, not necessarily the rest? Whoever it was, that hero had a big impact in this world's culture that is still felt to this day. Even if other heroes were, I don't know, German or Brazilian, the Japanese name for Earth would be set in stone.
Hmmm, now I noticed that I said "every hero", so I guess, yeah, there could be something like that, but based on the typical isekai trope, every isekai only affects Japanese people, it can only affect foreigners if they are in Japan at the time as exchange students in the isekai'd class, or if they are half-japanese in the first place.
Plus, other heroes would simply use the term "earth", "Terra", "Erde", etc... (
https://en.wiktionary.org/wiki/Earth#Translations )
As you suggested, "but the 1st hero was super popular, so everyone only remembers the name 'Chikyu', and ignored all the other later heroes from other lands", although in a way, that could be considered quite disrespectful.
Plus, another context to be considered is: Usually these novels and mangas are made for the Japanese public, not for a global public. The amount of manga that are never localized is quite significant. So it's easier to accept by default "all Heroes are Japanese" than try to add a global context of "all Heroes came from all around the globe".