I learned that there is an English word that indicatesfor the upper part of the legs of a person who is sitting down.

Rather than "that indicates," you can instead use "that describes"

"I learned that there is an English word that describes the upper part of the legs of a person who is sitting down."

I was convincThat is how I learned why portable size computers are called laptops.

"convinced" is more to do with persuasion and subjectivity. Someone can be convinced in an argument. Someone can be convinced by an advertisement. "Laptops" being called "laptops" because they refer to our laps is a fact. Therefore, it is not something you can be convinced of, but it is something you can learn about.

Also, "portable size computers" is incorrect usage of those descriptors. "Portable computers" is the correct way, same with things like "portable freezer," "portable games console," "portable fan".

They can be used by putting them on your lap.

This word can't be translated into Japanese, so the Japanese hardly usedon't say "laptop" and usebut say "note PC."

"Hardly" implies that something can be done, but isn't done often. Since "laptop" can't be said in Japanese at all, the adverb doesn't work here.