It's likely a difference between Cite book on en wiki and the template that is used on ar wiki for books?

I think we need to see the source code of the template like {{cite book|title=|author1=|first1=}} in english and arabic

In Arabic Wikipedia

<ref>{{cite book|title=African Boundaries: A Legal and Diplomatic Encyclopaedia|url=https://books.google.dz/books?id=A8Du4k0udx4C&pg=PA647&dq=d%C3%A9partement+chari+baguirmi&hl=fr&sa=X&ved=2ahUKEwij_-28mNTrAhWK3YUKHdCXC5UQ6AEwA3oECAQQAg#v=onepage&q=d%C3%A9partement%20chari%20baguirmi&f=false|publisher=C. Hurst|date=1979|ISBN=978-0-903983-87-7|language=en|author1=Ian|first2=Ian R.|author2=Burns}}</ref>

In English Wikipedia

<ref>{{Cite book|last=Brownlie|first=Ian|url=https://books.google.dz/books?id=A8Du4k0udx4C&pg=PA647&dq=d%C3%A9partement+chari+baguirmi&hl=fr&sa=X&ved=2ahUKEwij_-28mNTrAhWK3YUKHdCXC5UQ6AEwA3oECAQQAg#v=onepage&q=d%C3%A9partement%20chari%20baguirmi&f=false|title=African Boundaries: A Legal and Diplomatic Encyclopaedia|last2=Burns|first2=Ian R.|date=1979|publisher=C. Hurst|isbn=978-0-903983-87-7|language=en}}</ref>

What is the specific issue you are identifying with this citation? Is it:

1. English does not display "in English" anywhere?
2. The authors are different?
3. Something else?

The first is by design. Citations with lang=en only do not display the annotation. (lang=en, de would display (in English and German)).

2. The authors are different?

Yes, per arwiki user complain

In T263632#6702707, @alaa wrote:

2. The authors are different?

Yes, per arwiki user complain

So this is probably down to your template data and or how your template works: https://ar.wikipedia.org/w/index.php?title=%D9%82%D8%A7%D9%84%D8%A8:%D8%A7%D8%B3%D8%AA%D8%B4%D9%87%D8%A7%D8%AF_%D8%A8%D9%83%D8%AA%D8%A7%D8%A8/%D8%B4%D8%B1%D8%AD&action=edit

If you look at the citoid maps there, it sets author this way

"author": [
    [
        "author1"
    ],
    [
        "first2",
        "author2"
    ],
    [
        "first3",
        "last3"

etc.

But on en wiki it's

"author": [
    [
        "first1",
        "last1",
    ],
    [
        "first2",
        "last2"
    ],
    [
        "first3",
        "last3"

To fix this I've submitted a change to the template data.

In T263632#7061501, @Mvolz wrote:

To fix this I've submitted a change to the template data.

Thanks @Mvolz. Ofc your edit will not work, as you made it on the documentation page, not on the template itself.

Arwiki citebook template invoke citation/CS1 module, so I think you'd put your edits on the module, I'm correct?

FYI @Mr.Ibrahem