Textual: Mormon tells of Ammon reading the plates of Zeniff because they are the introduction to the question of reading other plates. The plates of Zeniff are the bridge in the action between Ammon’s interest in Zeniff/Limhi and this new question of unread plates.
We may reconstruct some of Mormon’s source here. Even though abridged, there is enough of the skeleton and the logic to understand what the original described. After the public discourse, Ammon is brought to the king’s residence, and as the continuing building of mutual history between Limhi and Zarahemla, Ammon reads the history of Zeniff and his people. It is significant that this history is on plates, because the idea of plates triggers another issue for Limhi. His people have found plates that they cannot read. Triggered by a foreigner reading plates, Limhi thinks of the foreign plates he cannot read.
The question Limhi asks is whether or not Ammon can interpret languages. The implicit jump from Zeniff’s plates to the discovered plates can be reconstructed from what comes later, but it is not apparent here. The transition for Ammon must have been abrupt. When Ammon finishes reading, he is asked if he can interpret languages. This is not a clean transition in writing, but a very logical one for a live interaction. We may presume, therefore, that Mormon’s source recorded the action with reasonable faithfulness, and that the question of interpretation preceded the explanation of the question in the original. Even in his close editing, Mormon is preserving the sequencing of the original.