English and Japanese

It is said that Japanese people are no good at English.

There is more than one "Japanese person" so we need to use "are".

One of the reasons is that that word order of English is too different from Japanese.

Once we talk about "word order of English" as a class, there is only one (class). So in this case, it's singular "is".

So I think it's very very hard for you to learn Japanese!

It is said that Japanese people aren't good at English.

One of the reasons is that the word order of English is very different from Japanese.

So I think it's extremely hard for you to learn Japanese!

It is said that Japanese people is are no good at English.

"Japanese people" is plural, so "are" here rather than "is".

Why (might this be)?

This is fine on its own but may be seen as a bit blunt! "Why might this be?" would sound better (I think in Japanese it's like saying なぜ？ instead of なぜでしょうか？)

One of the reasons is that the word order of English are is too very different from Japanese.

You're being specific about the aspect of English that you're talking about, so don't forget to use "the".

"Word order" is singular. We use "too..." when we think things have become excessive - a bit like Japanese use 過ぎる I suppose. So, to use "too" here would sound unnatural - "very" is better.

So I think you must work very very hard if you learn Japanese!

Sadly this doesn't make much sense in English!

I think what you're trying to say is the English equivalent of 頑張る. In that case, "I think you must work very very hard if you learn Japanese!" The "if" is important because it is not certain that everybody does learn Japanese.

It is said that Japanese people are not good at English.

Why is that?

One of the reasons is that word the order of English is so different from Japanese.

So I think it is very very hard to learn Japanese!