While we don't have any direct written records of what she spoke, historical context and linguistic evidence strongly suggest Brittonic as the most likely language.
It's important to note that:
* Brittonic was likely diverse: There were various dialects of Brittonic spoken throughout Britain, so Boudica's language might have differed from what is spoken in modern Wales, Cornwall, or Brittany.
* Latin influence: The Roman Empire had a significant presence in Britain during Boudica's time, and some Latin influence on her language is possible, especially in official contexts.
However, Brittonic remains the most probable language she would have spoken as the native language of the Iceni.