"This bento is good."
For your own usage, just remember to definitely add "o"/"go" if whoever you're talking to had made the lunch in question or is otherwise responsible for its presence before you :) Some words even have synonyms for this instead of just getting o/go on them: say, "(my) son" and "(your) son" sound completely differently.
Plenty of English speakers know what a bento is. Just depends if they've ever been to a Japanese restaurant in their life. Same goes for lots of foods in other languages; nasi goreng, thali, kim chi, paella, etc. It doesnt make sense to translate most of these. People can just google it if they don't know the word.
If it has a wiki, I think it's standard enough (though of course not all English speakers will know the word). "Boxed lunch" should also be accepted and seems to be the standard translation of "bentou". I will say that I would never use the phrase "boxed lunch" and am not even sure what it means outside of the context of being an English translation of "bentou".
は denotes topic and is really an unfamiliar case for native English speakers (at least for myself). My favorite translation is "as for -." In this case このおべんとは - "as for this bento (sounding proper)" おいしいです。"it is good." Then after another step, you might rephrase the sentence as something more natural, "as for this bento, it is good," - "This bento is good." The prevalence of this cased is pretty mystifying to me and I would imagine most native English speakers, because after the rephrasing step, it seems to take many different forms. を on the other hand is mostly consistent with the direct object, the thing an action is performed on. "I eat rice" - "ごはんをたべます。” However, there are some concepts that are conveyed very differently in Japanese compared to English. For example, "I like rice." In English, rice takes the same case as the previous sentence: the direct object. In Japanese however, the verb for "like" - 好き is different than in English. I think of 好き more like "to be liked" or "is pleasing." In which case, you can use が or は to make the sentence "ごはんが好きです。" - "(speaking for myself) rice is liked" which you would rephrase as "I like rice." Which might be the root of your confusion. In light of this, を and は are quite different.