Extracts can be found in English on the website.
You can find extracts in English on the website.
When writing this passive sentence as active voice what “in English” describes the same? Would it be correct if I use “as” instead of “in”?
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Both of these are acceptable, but the first example would be clearer and more strictly grammatical if rephrased as
Extracts in English can be found on the website.
"in English" modifies "extracts" and should ideally be placed right after "extracts" to make that clear. This is true whether a passive or active construction is used.
The placement of "in English" after "found" could be taken to mean that one searches in English as opposed to some other way, which does not seem to be the intent.
The repetition of "on" in the first example in the question is probably just an error/typo, but if it was intended, it should not have been.