Translation is always a tricky task and phrasing will vary between any given translator. However, a general principle that I prefer and believe produces better results is balancing transliteration (literal word for word translation, which rarely makes sense) and translation of intent. Balance is key.
So here, as an example, if the thought in English actually is "My condolences", but the equivalent Vietnamese phrase literally translates to "One sentence of condolence", then what is the better English version of the phrase to help an Eng->Vt learner understand when and how to use it? The first phrase, obviously! Never in my life have I said to someone who lost a loved one in death "One sentence of condolence". But I've said "My condolences" plenty of times, and that phrase, that idea, carries emotional weight that I now want to attach to the equivalent Vietnamese phrase so I can use it in the same situation, just in a different language.
THAT is what translation and learning a language is all about: transferring ideas and emotions from your native tongue to the new target language. Transliteration will not get you there.
Transliteration has to do with mapping sounds/letters from one language to another, not just translating literally. But yes, point taken.
I quite agree with your comments. This Duolingo offering makes me cringe too. But transliteration is most often the necessary step before translation. No pain no gain.
This sentence simply means "condolence" in English. However it is phrased as "A condolence sentence" in correspondence with individual words in Vietnamese. Please accept the odds in the English version and focus on the Vietnamese sentence.
If it simply means "condolence" in English, then it simply means "condolence," not "a condolence sentence," which does, indeed, sound like nonsense. If we are to focus on the Vietnamese sentence, then do not have us translate it into English, since the act of translation asks the translator to focus on both the Vietnamese and the English phrase.
No, I haven't never heard this before except in duolingo. I'm Vietnamese. I always say "Một lời chia buồn" or "Một lời chia buồn sâu sắc".