Translation:I want to improve my English proficiency.
I want to improve my English. This is the normal way we say this sentence in English. In Chinese 水平 is required to convey the meaning of proficiency level, but in English this meaning is implied.
Same problem. The hint also gives 'skill', so I think it should be accepted.
水平 literally refers to a level, not just to the proficiency. In all instances in this module the English translation should be "proficiency level" or "skill level".
"I would like to improve my English proficiency" marked wrong, but why, Duo?
"I want to improve my English proficiency" is bad English. No native English speaker would say this.
February 17, 2019: These sentences were not accepted.
"I want to improve my English."
"I want to improve my English skills."