The first one makes sense, although it's better as "Sentences, that are..., don't make sense." You could make it even shorter: "Sentences translated by Google Translate don't make sense". The second sentence is incorrect. "A sentence which is translated by Google Translate" is treated as a single noun, and is not a statement by itself. I'm not sure what 'it' was meant to refer to, but if you say "It doesn't make sense that..." then 'it' represents whatever statement comes after 'that'. Note that if you change what comes after 'that' into a statement, you're not saying that a noun (ie. "sentences translated by Google Translate) doesn't make sense but rather that the possibility of the statement being true doesn't make sense. Ex. It doesn't make sense that sentences translated by Google Translate make sense (this sentence isn't very natural, but the grammar's right).

The sentence, which was translated by Google Translate, doesn't make sense.

The sentence that was translated by Google Translate doesn't make sense.