When your main concern is use of は, the problem is the same as the は-が problem in general.
With は, it's a complete sentence by itself with topic これが正しい翻訳かは and comment わからない.
Without it, it's an incomplete sentence on its own without topics and only with comment これが正しい翻訳かわからない. Of course, however, people can guess what it's telling about from context. For example, if the previous line is 情報が少なすぎる (we have too little informaiton), a topic to be shared by the both lines will be, say, これでは (at this rate) or so.
Besides the problem of topic, in other words, whether it functions as a primary topic or not, は can imply the sense of "even if I know the others" in the example that says you don't know one. (Note that not "though" but "if".) This function is often called "contrast" or "contrastive marker は".
In addition, は can be added to a negative predicate like わからない just because it's a negative redicate even if it's not a primary topic or contrastive.