In all honesty with regards as to why pokemon repeat their names as sounds to which how they can seemingly interact with each other intelligibly cross-species is rather perplexing. On one hand, if you follow the compilation of pokemon lore, there are at least 3 different origins of pokemon, pokemon are either natural beings which were created with the rest of the world/universe from which follow pseudo-evolutionary lines with a core common genome which manifest itself as Ditto and found within the ancestral Mew. There are also artificial man-made/resurrected beings such as porygon, kabuto, and grimers. And there are spiritual pokemon which are said to be spirits of the dead sentients or inanimate objects ie Banette/drifloon/spiritomb/yamask/shedinja.  Thus curiously, the idea of egg groups makes some sense with regards to having a certain percentage of DNA to interbreed cross 'species' as the degrees of differences may actually be considered 'minor' relative to the core genome in itself. However since it is maternal true-breeding, there must be some sort of differentiate in the genomics that lead to heavy maternal imprinting or so to determine the offspring 'species' thus all pokemon are actually all related to each other and that the differences in pokemon core genome in itself to explain the genetics are actually minor in comparison. 

As a really bad example, Wailord and Skitty are capable of breeding, and Skitty is compatible with Shroomish but Wailord cannot breed with Shroomish. Wailord however can breed with magikarp, and magikarp can breed with treecko, and treecko can breed with bulbasaur, and bulbasaur can breed with shroomish which then can breed with skitty. Hence, given that most pokemon are in at least 2 egg groups, there must exist a core of genomic similarities that allow pokemon to breed in this manner despite gross anatomic variations. Don't even get me started on Nidoran Reproduction.

Thus begs the question what about ditto? Well ditto can breed with almost anything. Perhaps by being a genetic donor, which supports the theory that all things, even man-made pokemon and 'spirit' pokemon share the same genomic activity with relatively small variations. Thus rises the question if pokemon and humans could be technically related to each other via this theory? Which is to say that since pokemon such as deoxys form from space DNA, is it possible that everything in the universe shares a common genomic DNA exploited by pokemon egg groups? And as such, pokemon are actually just another branch point in the evolutionary line wherein humans are just another type of pokemon who communicate with each other and have lost the ability to specifically communicate with pokemon by no longer saying our names, but instead say other things dependent upon the names of other pokemon?

"Magma" is called magma because "magmar" live in it. Gravel is called gravel because "Graveler" look like it. A snore is called a snore because "Snorlax" make them. And so forth, filling the gaps with inventive words when necessary.