Japanese readings of 唐
唐 is Ch. Táng, the dynasty that ruled China from 618–907, and is widely considered a 'golden age' of Chinese civilisation (insert hermeneutic suspicion about historiography). It was during this time that Japan started adopting loads of practices and systems from China, and so 唐 enters as one of many sinograms in Japanese for China.
The go-on どう and kan-on とう are pretty straightforward, but when we get to kun readings it gets weird.
We first have から kara (which is the Jōyō kun reading) : から was apparently also used to refer to Korea, and so apparently it may come from the name of the Gaya 가야 加倻 confederacy in the south of the Korean peninsula. There are also apparently alternate transcriptions of Gaya into sinograms that may suggest something like kara or kala for the original pronunciation of Gaya. I haven't actually read any sources other than a few websites, but this strikes me as plausible. Then we have this truly bizarre もろこし morokoshi, which is apparently a native reading of a Chinese collocation 諸越 (cf. 諸々 moromoro; 越す kosu); the few webpages I've read parses this as "various Yue [peoples]", 越 Yue being an ancient state in the South of China and thereafter a generalised term to refer to the South. I'm not sure where the historical links are, i.e. was there a lot of contact between the South and Japan? Or was it through intermediaries? Or maybe morokoshi is actually ateji or a different reading?
Btw, I stumbled into this when I found the word 唐船 with the reading もろこしぶね. I KNOW