Many fans of Dragon Ball Z make the mistaken assumption that the Namekian language was introduced in the "Namek Saga." This would make sense, as the concept of Planet Namek wasn't formally introduced until the "Saiyan Saga" one arc prior. However, while the planet wasn't mentioned until this arc, the language and its speakers were both introduced a little further back.
The first instance of Namekian being spoken is presumed to be Moori speaking to Frieza, but it's actually a conversation between Piccolo and Kami. They had this conversation one arc prior to the "Saiyan Saga" during the final arc of the original Dragon Ball anime. Bulma even mentions this at the start of the "Namek Saga." Here's an overview of what happened and what was said in this easily-forgettable moment in Dragon Ball history.
It all started at the 23rd World Martial Arts Tournament. At the time, Piccolo was better known as the evil spawn of Demon King Piccolo. He entered the Tournament under the alias Junior so he could have his chance at revenge on Goku for slaying his father. Kami also entered the Tournament in an effort to stop Piccolo's evil once and for all; he hid his own identity by taking over the body of an unassuming middle-aged man name Hero (Shen in the Japanese version). Naturally, these two powerful warriors made it past the preliminaries and faced each other in the second match of the semifinals.
This fight was where the conversation took place. Unlike in DBZ where Namekian is spoken plainly, the language here was dubbed over with regular human language while Namekian text flashed across the screen. This was likely done for the sake of convenience for both the anime staff and the audience. Unfortunately, nobody at the Tournament could understand what these two were saying; from their perspective, the two fighters were speaking an alien language.
The conversation was brief but meaningful. Piccolo revealed that he realized Hero was Kami disguised as a human being, which led Kami to continue their conversation in Namekian. This was likely so he wouldn't be outed as the Guardian of the Earth in front of everybody. He told Piccolo how, despite the fact that their fates were intertwined, he fully intended to stop him however he could. Fortunately, he revealed that he didn't have to kill Piccolo to stop him, as he'd brought a bottle that he could use in tandem with the Evil Containment Wave to seal the villain instead.
The real impact of this conversation is how it's one of the few early signs that Piccolo and Kami aren't from Earth. Before this arc, Piccolo was presumed to be a demon while Kami was presumed to be God. It wasn't until this arc that the two were confirmed to be aliens.
This moment goes largely forgotten even in the story of Dragon Ball. The only time it came up afterward was at the start of the "Namek Saga" while Bulma and Mr. Popo were looking for the ship Kami used to get to Earth. Even then, it was only mentioned for a moment and it was easy to miss. If anyone forgot about this little moment or simply never knew it existed, it shouldn't be held against them.
