Adding on to the previous answer, I could never have imagined that I would learn Chinese someday. I never imagined that I would learn Chinese someday. The sentence you wrote can be grammatically correct but that would not be something that you'd want to convey. The meaning of your sentence is that you would never imagine in future that you would be learning Chinese. That obviously is not what you meant. Right ?

Now I feel ashamed to have asked this question. How can I remove that :)

I could never imagine that I would learn Chinese someday. I wouldn't ever imagine that I would learn Chinese someday.