What culture do we have the most written records of which are in a language which has not yet been deciphered?

I would think the Bronze Age Indus Valley civilisation is the best candidate, it was based within present day Pakistan, Afghanistan and North-West India. It's appearance in the archaeological record is contemporaneous with pre-dynastic ancient Egypt and with the first cities of ancient Mesopotamia.

We can tell that they were a sophisticated society as they built cities, traded with their neighbours, constructed sanitation and irrigation infrastructure, produced arts and crafts and so on. To achieve these things they must have had social organisation to work collectively and they must have had some time and resources in surplus for non-essential activities.

There are hundreds of symbols that have been discovered within inscriptions on their artefacts, but we don't even know which family their language belonged to, whether those symbols stand for words, a syllabary, or indeed whether they're writing at all. Scholars have suggested a move from pictographic to alphabetic writing over time as we see with other writing systems of the region, but without being able to decipher the script that's speculative.

Having access to their records would open up a wealth of information on the formative period of the earliest 'Old World' civilisations, one only has to consider the breakthroughs made with regards to translations of other ancient texts of the region such as Sumerian to appreciate the potential. There's also tantalising glimpses of symbols that may be related to ancient Indian religious traditions.

Discovering traces the older Indian Religions or belief systems which fed in to the creation of Hinduism, the world's third largest religion with around one billion followers, would enrich our understanding of World History immensely.