同意刘老师的解答，若将后面的the English改为that，那么前面必须是the English，而不是old English。

这与你在追问中提到的The weather of China is quite similar to that of the US.是不一样的，前面的the weather有of China作定语，后面的that（=the weather）有of the US做定语，也就是说，that只是代替前面的the weather而不是the weather of China。

▲如果你的原问题中的old English变为the English they spoke in ancient times则后面可以用that代替the English。即：

The English they spoke in ancient times is very different from that we speak nowadays.

▲就此机会说说it与that用作指代时的不同用法：

it表“同一事物”；that表“同类事物”，它们的复数形式分别为them和those。比较：

The book must be there, or else you've lost it. 书一定在那儿，要不然就是你把它丢了。（it就是前面的the book，是“同一”）

These trips were strenuous, and the couple did not enjoy them. 一路舟车劳顿，这对夫妇玩得并不开心。（them是前面的these trips，是“同一”）

The weather in Guilin is quite different from that in Beijing. 桂林的天气与北京的天气大不相同。（两地的天气是“同类”，不是“同一”）

The squids are bigger than those I bought last week. 这些鱿鱼比我上周买的大。（两种鱿鱼是“同类”，不是“同一”）