According to Wikipedia Akkadian cuneiform was considered deciphered after 4 scholars translated a previously unseen text and the translations were similar. This strikes me as circular logic. If these scholars were basing their translations off of the same (possibly erroneous) ideas about the vocabulary and grammar of the written language, then they would arrive at similar translations even if those ideas are wrong. It seems to me that this “similarity test” is flawed. How confident are we that the writing was correctly deciphered?
The simple answer is that a "Rosetta Stone" exists in the form of the Behistun inscription made by the Achaemenid Great King Dareios - it has the same text in Old Persian, Akkadian and Elamite. /u/Bentresh describes the process here: https://www.reddit.com/r/AskHistorians/comments/7crp0n/when_did_the_modern_understanding_of_the_history/dq5tf5e?utm_medium=android_app&utm_source=share
A few added comments on the Behistun Inscription's characteristics: Various conventions make the process easier, like the use of special characters (ideograms) for words like "King" and "Ahuramazda", which allows you to infer that e.g. the king's name, the name of his dominions, parts of the religious vocabulary etc are near those characters. And we can get a good idea of what Old Persian should look like by comparing to Sanskrit, Avestan and Middle Persian. Much of the Behistun inscription's story is also retold by Herodotos and its illustrative reliefs.
If you have any followup questions about the Behistun inscription in particular, let me know.