English Translation
It is hard to understand what he is thinking.
Unnatural a bit although understandable. Here are my alternative translations.
彼の考えは分かりづらい。（かれのかんがえはわかりづらい）
彼の内心は読みづらい。（かれのないしんはよみづらい）= (lit.) It’s hard to read his mind.
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