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"Peng frapo moe plltxe na'vi," translated, means (literally), "Tell everyone we speak Na'vi."

It could also mean - depending on the meaning behind the 'Na'vi' terminology - "Tell everyone we speak (for the) people." Na'vi, translated, can stand for either the Na'vi language itself or 'the People' in their own words.

In this situation, as there is no distinguishing phrasing mentioned to suggest the latter, the sentence likely, in the end, stands for, 'Tell everyone we speak Na'vi."