I don't think English is as difficult a language as Russia

我来回答第一个问题。as...as 是个短语。中间可以加上形容词等，意思是与.....一样，如as tall as . he is as tall as me .

是 as difficult 像。。。一样难
你说的的in which 是用在定语从句里的 表示后面引导的是地点 in which =where
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应该是as difficult,因为在as之后，应把形容词、副词等提前，再加名词，as...as意为“和...一样”。

正确说法：I do not think English is as difficult as Russian.
已经知道是在说语言了，就没有必要重复，另外加language.

你能不能把你的第二个问题说得更仔细一点。我不太明白您的意思。

in which 只能用在定语从句中，等于where,在定语从句中作状语。
如：He lived in the house in which Tom once lived.
He lived in the house where Tom once lived.

as。。。。as 是和什么一样的意思，可以作为一个常用词组记住。这句话的意思是我不认为英语像俄语一样难。
第二个问题最好你还是有例句这样我可以看一下