Maybe it's just me, and I would need to ask some other Swedish native speakers what they think to be completely sure, but I would not say "Det är grekiska för mig". While the sentence is not exactly wrong in itself, it's not what people say when they want to say the equivalent of "It's all Greek to me". Not in my experience anyway. To me it looks just like it word for word translation of the English saying.

Since it's questionable that a native speaker would ever say something like this, I'm adding the "tag" needs native check to this sentence.

@needs native check

Yes, I'm leaning towards agreeing with that being the best, i.e. unlinking the sentence. Or even deleting/changing it entirely. But even though I'm a native Swedish speaker with a passion for languages, I'm not 100% sure that other Swedes would agree with me that the sentence sounds unnatural. I provided the saying that I hear: "Det är rena rama grekiskan". The word "rama" is sometimes omitted. And "för mig" might be right/acceptable, but I don't use those words to end this expression.

I totally agree with malinef. The saying is “Det är rena grekiskan.” The word “rama” is sometimes used for extra emphasis. In my opinion that’s enough to translate the English “It’s all Greek to me.” If, for some reason, the context would require the “for me” part to be translated, I’d say that the natural wording would be: “För mig är det rena grekiskan.”

I'm glad you agree with me! In this saying, the word grekiska must be used with the definitive article (thus "grekiskan" (the Greek [language]), and not just "grekiska" (Greek)), or else it doesn't give the right ring to it to a native Swedish speaker. I also agree about the wording that you mentioned. "För mig" might start the sentence, and while it's not wrong to end the sentence with it, it doesn't sound as natural.