I'm no expert on languages, but it seems like it is vastly harder to decipher language isolates. For instance, even if you know what sounds the various signs make, you would then just have a long list of meaningless sounds. In fact, isn't that the current problem with deciphering Etruscan?
So when I was looking up information on how Sumerian was deciphered, every source just lumps it together with the deciphering of Akkadian. But logically even if you knew what sounds the different cuneiform symbols recorded, how would you know what the Sumerian inscriptions were trying to say?
Part of the reason for this is that it started from the decipherment of Akkadian-early scholars found texts in this non-Semitic language(although even before these texts were found, some people suspected that cuneiform was not invented to write a Semitic languages because it is not well suited to them). While this was an isolate, some things were clear about it-based on the pattern of syllables it was agglutinating, and in addition some bilingual texts had been found, providing at least a useful start. Unfortunately, at this point progress was delayed by an extended feud over whether or not Sumerian was ever a real language or just a sort of argot for a Semitic languages. This lasted some years, until it was agreed that it was at this point a real language.
Now how was it deciphered from there? The bilinguals helped, but equally important, a staggering number of Sumerian texts were excavated in the 19th and early 20th century, providing an enormous corpus to base reasearch on. Equally crucially, this corpus included bilingual word lists, student exercises, and other texts that are useful to decipherers.So by 1880-90 the first efforts to produce translations of these texts were published and by. 1910-20 scholars(mostly Germans) were starting to put together lexicons, grammars, and so on. Of course this is a very rudimentary summary but hopefully it gives you a good starting point.
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