What can be said with reasonable confidence is that he spent significant time in Indian Territory and that he developed working knowledge of tribal languages and customs that was functionally useful to his subsequent career. The specific nature of those years — how long exactly, where precisely, with which bands or communities, what he did and learned and experienced — is documented primarily through later accounts rather than contemporaneous records. He was, during this period, a man without a formal legal status in the territory, living within Indigenous communities whose hospitality was a matter of practical necessity for him.
The tribes he lived among were not abstract categories. The Seminole Nation, in particular, had a complex relationship with Black people that was different from the relationship most white Americans of the era had. Seminole society had included Black members — some enslaved, some free — for generations before the removal, and the mixed communities that had developed in Florida had reconstituted themselves to some degree in Indian Territory. The Seminole were among the more egalitarian of the Five Civilized Tribes in their treatment of Black members and Freedpeople, though this is a relative description and should not be understood as complete equality in any modern sense.
What Bass Reeves gained from his time in these communities was more than linguistic skill. He gained an understanding of the land that could not be acquired from maps. He gained a network of relationships across a territory that would otherwise be foreign to him. He gained the ability to navigate between communities that most outsiders could not navigate between. All of this would define his career as a deputy in ways that official training, had such a thing existed, could not have replicated.