"Ellos necesitan los celulares ahora."
Translation:They need the cell phones now.
I'm only guessing but the two sentences "They need THE cell phones now" and "They need cell phones now" have different meanings. In the first case, it sounds like they already own cell phones and they need them. In the second case it sounds like they need to go buy cell phones. That's my thought process for what it's worth.
You are absolutely right. However, it's my opinion that we don't have privy to the context of these statements. Until Duo provides context we just have to guess their thoughts. My question is how does one translate into Spanish "They need cell phones now.
I left out "the" and Duo said I needed the article "the." Maybe translating from English to Spanish may require certain articles. However, translating from Spanish to English does NOT require articles. Since we don't know if cell phones need to be purchased or if they already have cell phones then it's difficult for many to answer correctly.
The sentence here is talking about specific cell phones. You need the article in both languages. If it were "They need cell phones now" in English, it would be just as article-free in Spanish: "Necesitan celulares ahora."