"Learning a new language is necessary and that is always an advantage."
Translation:Việc học một ngôn ngữ mới là cần thiết và điều đó luôn là một lợi thế.
Given that "Điều đó," is from a section called "relative clauses," I think the point is to distinguish between to different types of uses of "that" in English. We use "that" both to indicate an object in physical space--"that kid (over there)"--and to reference something previously discussed in conversation--"He said he was taking the day off, and I said that was unacceptable." The "điều đó" from this lesson is the latter case. I may be entirely off, but that is my working impression of the difference.
I don't think so. Đang học indicates something you are currently doing: "What are you doing? I am learning (Tôi đang học)."
In the English sentence above, learning is used as a noun: "The act of learning a new language is necessary." No one is currently learning in the sentence. Việc học is how you talk about "the action of learning".
So even though the word "learning" looks the same in English, it has a very different meaning and role in the sentence. Vietnamese, and most languages I know use different words, to distinguish these two roles.