Translation:Would you like a bento?
I don't think bento needs to be, or rather, should be translated. Bento itself is a very specific thing, that doesn't really exist anywhere else and most people - especially people taking a Japanese language course - are going to know what a bento is. It simply doesn't need to be translated. Moreover, where I'm from, if you asked me if I wanted a lunch box, I would assume you are asking me if I wanted a container to put my lunch in.
I feel the same way about お姉さん and お兄さん those two terms are something intrinsic to Japanese society, most people will know (or very quickly figure out) what it means, and it just sounds incredibly awkward when translated. Most good translators don't bother translating it, and it's easy to see why - I have a brother, and not once in my entire life did I ever call him 'big bro'.
I am so glad that you posted this. I have only been doing the Japanese course for about four months now, but everytime I saw bento translated as lunchbox it felt odd. When i saw the kanji for it I translated it to bento box. People like you change and improve Duolingo. I would never call a Bento a lunchbox. Lunchbox has a completely different meaning to me.
They might be able to infer the same meaning but there is a distinct difference. One is offering, the other is enquiring about desire... If you can differentiate with two English sentences then you can obviously differentiate between two Japanese sentences. Think about literal meaning behind words you learn. Once you understand those then you can start speaking colloquially.
Because a bento is a bento, in the same way that sushi is sushi. There is no translation for it. In English it's also a bento, or a bento box. In Hawaii, for example, it's quite common for grocery stores to have "bento boxes" and that's what they're called in English.