Why is Cristiano Ronaldo known as “C罗” in Chinese?
C for Cristiano
罗 for 罗纳尔多 (Ronaldo)




Apparently before him:


Ronaldo (罗纳尔多) (Luís Nazário de Lima) was known as 大罗
Ronaldinho (罗纳尔迪尼奥) was called 小罗


So, with big (大) and little (小) already taken, the first letter of his first name was given to him instead.



As for 朗 vs. 罗:

It seems that 朗 is Cantonese (Hong Kong, Macao) while 罗 is Mandarin (Mainland, Singapore, Taiwan).