*[ ] parts are where I got lazy and let ChatGPT finish for me ¶
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Religion is a somewhat foreign concept in Japan.

There are many customs in Japan that have their roots in either Buddhism or Shintoism, but most people don't consider them as religious practices—at least, they are not really aware of it.

So are, too, do the concepts of "monotheism" and "polytheism." share similar roots.

So are the concepts of monotheism and polytheism *what*?

Are you trying to communicate that these concepts also have their roots in Buddhism and Shintoism?

Anyone who has a moderate understanding of Shintoism knows that Shintoismit is polytheistic, or a religion that has more than one god.

"that has more than one god" is fine.

You could improve the English by using a word like "worship". For example:
"...., or a religion that worships more than one god."

It doesn't mean that there are exactly eight million gods (or "spirits," depending on how you translate the word "kami"), but "eight million" was used in Old Japanese to mean an "uncountably mane quantity" because, apparently, Japanese people back then didn't know the concept of "infinity," and number eight was considered a lucky number in East Asia.

NThe number eight appears in other legends, among themincluding the legend of Yatagarasu.

Yata means "eight-span" and "karasu" (garasu in certain compound words)" means "a crow," so Yatagarasu means "eight-span crow."

I'm not sure what "span" means in this context.

Based on this (https://yokai.com/yatagarasu/), a span was a way of measuring something. You might want to make that clearer to help the reader.

Yata refers to eight of whatever unit of measurement was used in ancient Japan, but it doesn't mean exactly eight of it.

I don't understand this sentence, I'm afraid.

I don't get how something can refer to eight but not exactly eight??

Curiously enough, this mystical crow has three legs, because odd numbers, too, are considered lucky too in Japanese culture.

It is an event for parents to celebrate their children turning 3, 5, and 7 years old.

In my opinion, they are not only propitious numbersnumbers of good fortune but also rhythmic.

The word "propitious" is appropriate here but not very common. I personally wouldn't use it as I think something like 80%+ of native speakers would not know what this word means.

Let's go back to story of the crow story.

So, when does good luck come in handy?

(Not being disrespectful to "football" people.

だいじょうぶ

We are very used to foreign countries referring to football as "soccer".

There are many customs in Japan that have their roots in either Buddhism or Shintoism, but most people don't consider them as religious practices—at least they are not really aware of it.

In fact, there is a Japanese expression that encapsulates the characteristic of Japan's indigenous religion: Yaoyorozu no Kami (eEight mMillion gGods).

This is mostly a stylistic thing, since you introduce Yaoyorozu no Kami as a proper noun, I think it's best if you capitalize it in parentheses. But every, for every following mention of it, it's fine if it's lowercase.

It doesn't mean that there are exactly eight million gods (or "spirits," depending on how you translate the word "kami"), but "eight million" was used in Old Japanese to mean "uncountably many" because, apparently, Japanese people back then didn't know the concept of "infinity," and the number eight was considered a lucky number in East Asia.

NThe number eight appears in other legends, among them the legend of Yatagarasu.

Yata refers to eight of whatever unit of measurement was used in ancient Japan, but it doesn't mean exactly eight of it.

Curiously enough, this mystical crow has three legs, because odd numbers are considered lucky tooas well in Japanese culture.

"... lucky too" isn't gramatically incorrect, but feels kind of clunky.

It is an event for parents to celebrate their children turning 3, 5, and 7 [years old].

Another example is [in] Japanese poetry.

Similar to rhymes in the English-speaking world, haiku and tanka sound comfortable to Japanese ears, so you could say that these numbers also have a musical nature in spoken Japanese.

Alternatively, you can phrase it to say "... these numbers sound musical in spoken Japanese".

Japan's national soccer team is called the "Samurai Blue" and their jerseys have an interesting logo on their chest—which is, you guessed it, Yatagarasu.

It is also a symbol of the JFA (Japan Football Association, which I think is a misnomer considering how widespread the term "soccer" is in Japan compared to "football").

I find it cute that the mystical crow conveniently puts his third leg on the ball, but one cannot help but wonder if it might violate whatever rullaws are stipulated in the international football matches.

It's weird, but the "rules" in soccer are officially called the "Laws of the Game"