Here's what we know:
* No written records: The Iceni didn't leave behind any written records in their own language, making it difficult to definitively determine what they spoke.
* Roman influence: By the time the Romans arrived in Britain, the Iceni were already under Roman influence. This means that they might have adopted some Latin words and phrases.
* Linguistic connections: Archaeological evidence and linguistic analysis suggest that the Iceni's language was similar to the languages spoken by other Celtic tribes in Britain, including the Britons who spoke Brittonic.
Therefore, while we can't be certain, it is generally accepted that the Iceni spoke a Brittonic language, similar to the language spoken by the Britons of the time, which later evolved into Welsh, Cornish, and Breton.