How do you say this in Japanese? For this sentence, wouldn’t で be more optional than に before 翻訳?

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Good question. I think 翻訳する has a nuance of into like "translate A into B."
で in this sentence is used to describe "means" "steps" as in,
私はこの文をGoogle翻訳で英語に翻訳する I translate this sentence into English by/using Google Translate.
On the other hand, 話す doesn't have function of "into" like speak A into B. so we don't say 英語に話す.
英語で話す is more like speak by/using English. or 英語で話してください。Please speak in/by/using English cf.) Please speak English. is 英語を話してください。 but sounds a little unnatural so we usually say 英語で話してください。