The English people are said to love tradition/ custom.

  1. The English people are said to love tradition/ custom.

  2. Kim speaks English more fluently in the science class.

  1. I would tweak that ‘The English people‘ part. To me it sounds as if you meant some English people you were talking about previously. “English people\The English” (= all English people) would be more appropriate here in my view.
  2. Hard to catch the idea: sounds like Kim is being fluent when only placed in the science class. I would expect something like, “Kim speaks English more fluently than the rest of the science class.”

Won’t you need the plural - ‘traditions’ and ‘customs’, Eugene?

I was considering that too. To me the choice could depend on what you meant to say.

Thank you for your help.

My trouble here is we should use " custom or tradition" in this case. I don’t know please help me.

As I see it, they are close, and not so easy to distinguish.
‘Custom’ could denote established behavior; convention (what is considered acceptable or polite to most members of a society) —Custom demands that a gentleman take off his hat when a lady enters the lift.
‘Tradition’ could mean a long-established or inherited way of thinking or acting; traditional ways — According to tradition, you should let a cat in first when moving in.

In your case, ‘tradition’ sounds more general to me, meaning the English hate to break traditional ways of doing things which their predecessors once did.