Bento is not an English word per se, but is a loan word from Japanese. It's also not in particularly common usage, so you may expect some quizzical looks by native English speakers who are not familiar with Japanese cuisine.
In direct answer to your question though, the answer is Yes, Bento can be used in isolation, as can be seen on the wikipedia entry for Bento:
Bento (弁当 bentō?) is a single-portion takeout or home-packed meal common in Japanese cuisine. A traditional bento holds rice, fish or meat, with pickled or cooked vegetables, usually in a box-shaped container. Containers range from disposable mass produced to hand crafted lacquerware. Bento are readily available in many places throughout Japan, including convenience stores, bento shops (弁当屋 bentō-ya?), railway stations, and department stores. However, Japanese homemakers often spend time and energy on a carefully prepared lunch box for their spouse, child, or themselves .
Bento can be elaborately arranged in a style called "kyara-ben" or "character bento". Kyaraben (キャラ弁?) is typically decorated to look like popular Japanese cartoon (anime) characters, characters from comic books (manga), or video game characters. Another popular bento style is "oekakiben" or "picture bento". This is decorated to look like people, animals, buildings and monuments, or items such as flowers and plants. Contests are often held where bento arrangers compete for the most aesthetically pleasing arrangements.