Hi, neither of those sentences sounds 100% fluent - try these instead:
What is hard for you when learning English?
What is the hardest part of your English studies?
What do you find challenging about learning English?
What do you find hard about learning English?
In English, "we find it hard" if something is hard. If something is interesting, "we find it interesting"; if the food tastes delicious, "we find it delicious."
I hope this helps.
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