Most of the meanings of {'ach} express some kind of contrast, but Klingon collapses all the various nuances of the English words ("but", "however", "nevertheless", "whereas", etc.) into one word.

"I speak English, but he speaks Klingon" isn't significantly different from "I speak English, whereas he speaks Klingon." In most cases it probably won't be necessary to worry too much about what specific word will eventually be used to translate {'ach}.