I think (and I'm not exactly a leading expert in Japanese, so forgive me if I'm wrong) that the rank of Taishou is usually ranked as General in western army terms. However, the Bleach rankings aren't strictly army ones, and as 'Taishou' literally means something more like 'Unit Commander' which fits the Soul Society ranks just fine, it's used there and translated into English as 'Captain'.

I think, anyway. ^_^