It's all Greek to me. Could you try to explain more simply?
Please use simple words, I couldn't understand your explanation at all.
I don't understand a single word. It's too complicated to understand.
"Can you make it so even a child would understand?"
"Your are using words that are too difficult to understand, would you mind explain in a simpler way?"
etc.
I don't understand what they are saying. It's too complicated.
I don't understand what they are saying. It's too complicated.
彼らが何を言っているのか理解できない。複雑すぎる。
「頭のいい人」は特定の１人ではないと思うので、they で表すことになります。
Whenever I ask some question to smart people, their answer is too complicated.
頭のいい人に何か質問すると、いつも答えが難しすぎる。
Why they can't explain things in a simpler way?
どうしてもっと簡単な方法で物事を説明できないんだろうか？
「難しくて日本語を読んでるとは思えない」で一番適切なのは、他の方も書いている It's all Greek to me. でしょうか「私には全部ギリシャ語に聞こえる」という意味です。面白いですよね（笑）
普通に言いたいのであれば、
I can't believe I'm reading Japanese. It's so hard to understand!!
「日本語を読んでるなんて信じられない。理解するのが難しすぎる！」
フランス 
日本 
