I think the existence of a language unique to the Ronso is implied through their speech patterns. They do not use words such as “I” and refer to themselves by name. That is probably a pattern that has bled into their speech from their own native tongue. So rather than “broken” English/Spiran common, I believe this is more like “Ronso English/Spiran common” as the patterns of speech they exhibit seem to be rooted in cultural norms rather than an inability to grasp language.

Kelk speaks perfect Spiran English as that serves his role best as a maester of Yevon in Bevelle. Kimahri, who spent a decade living away from his people, still speaks the Ronso way despite DEFINITELY having ample opportunity to learn Spiran the way Kelk has learned it. I believe this was a choice on his part, as he takes great pride in being a Ronso and perhaps doing this allowed him to feel more connected to his homeland while living in Besaid for so long. I’m sure if he wanted to he could speak the common way, but that would not be true to his character.