Hi Bachir, “Weak in” English (as a subject of study like history or science), but “bad at” English (as in the language skills of listening, writing, etc).

He is weak at Maths. He is weak at sports. His English is weak. It sounds odd to say "he is weak in OR at English.' But for any other subject, we say, 'he is weak at...' However, we do say: 'He is weak in this area. He is weak in some subjects.'