It is actually hard to guess what 'ach is. Usually, when you go over a yellow word (if it is a new one) in a lesson, a meaning shows up, but when I got the new word 'ach, nothing showed as a translation in the bubble. I used context to find out what it means. Please put a translation or tell me why am I wrong!
We've added a translation hint -- but Duolingo's system doesn't display it.
Words that start with an apostrophe don't show any hints; this is a problem that Duolingo has to fix. We had hoped that it this would be corrected before we went into beta, but the problem still persists; it's just one of many problems that the developers have to work on and they haven't got to this one yet.
It's issue 2 on our list of known issues with the course: https://forum.duolingo.com/comment/26778885
Until Duolingo gets fixed, for words starting with an apostrophe, you'll have to learn them from context, from looking at the translation shown after you've done the exercise, or from looking up the word elsewhere. (If you have a mobile device, many people like the boqwI' app. And having a paper copy of The Klingon Dictionary is always a good idea.)