There appears to be a missing hyphen. The word is “alingualism” It appears in the dictionary though I had not heard it. The prefix ‘a-‘ often means ‘not’ or ‘without’

I have never seen the word. I think the word "definition" in the first sentence is a terrible word choice. A good translator is not "by definition" bilingual. I think what the author meant to say was that a good translator must necessarily be bilingual. The second sentence also does not make sense because the first sentence has no "opposite". The phrase "by definition" has no opposite. What would the opposite of "by definition" be? "Not by definition"? " "By not definition"? It's nonsensical. I think the author is imagining that the "opposite" of the first sentence is: "a bilingual person is by definition a good translator". That is such an preposterous falsehood that saying "it is not necessarily true" is a ridiculous thing to say.

In the text, they give you their definition of the word, "a state in which a person lacks a full, fluent command of any language." But while you'll find entries for words like "bilingualism" and "multilingualism" in any dictionary, that's not the case for this word. It seems to be a modern made-up word used by some linguists, and the ones who use it all define it in different ways. Don't become obsessed over defining extremely uncommon words. Your energy would be better spent on learning the basics like subject-verb agreement. In the same sentence with this word, you filled in #52 with "are" when it should have been "is." Getting that right is much more important than trying to clarify the definition of a made-up word that can't be found in dictionaries.