Forget who shot first! I need answers to THIS plot hole!

answer: he doesn't like Han Solo

i used to be a basic speaker, like you. but then i took a golf ball to the throat.

He chose not to in order to make a point

Huttese was the language spoken by the Hutts, a slug-like species who called Nal Hutta their homeworld. It was also commonly spoken on Hutt-controlled planets like Tatooine. Even on worlds such as Lothal, where Hutts were not in power, their tongue was still a widespread trade language. Despite its prevalence, Huttese was generally unknown to stormtroopers, the soldiers of the Galactic Empire, as most of them could not speak anything other than Galactic Basic Standard.

Greedo was speaking to Han Solo in Huttese, this is also the language Anakin spoke as a child to Sebulba.

To further augment the argument, in Holographic barricades, tape at a crime scene on Plazir-15 in Mandalorian Chapter 22, it read "CAUTION" in Aurebesh. And "CHESS KO"* (Huttese for "careful"; as in "be careful") in Outer Rim Basic, suggesting that the language was spoken there as well.

* Anakin: "Chess Ko, Sebulba, [in Huttese] He's a big time out-of-towner..."