Firs of all, it's: I won't be able to translate this text. The first is future tense, the second is past tense. The first is saying that, without trying, you already know you will not be able to do it. Ex: I'm busy all weekend. I won't be able to translate this text. After looking at it for ten seconds, I realized that it was way beyond my level. I won't be able to translate this text. The second implies that you tried but were unable to translate the text. Ex: I worked on it for two hours, but it was just too difficult. I could not translate this text.

There isn't much difference between them. If the meaning is managed to or succeeded in doing, use ”was able to” and not ”could