>> most dialects have completely different sets (i.e. non-1-to-1 mappings) of words than Mandarin, and many have supplemental character sets that wouldn't make

>> most dialects have completely different sets (i.e. non-1-to-1 mappings) of words than Mandarin, and many have supplemental character sets that wouldn't make any sense to someone reading Mandarin Yes, that is correct. Cantonese is probably the non-Mandarin dialect with the most writing going on, and that is mostly informal writing like facebook, blogs, gossip magazines, although there is some modern literature written in Cantonese characters, wikipedia in Cantonese probably counts as well for more formal writing. >> The Hong Kong Supplementary Character Set, for instance, is a set of characters that are specific to the Cantonese dialect, and don't have close/exact relatives in standard written Mandarin Yes, though I don't know if there are Unicode code points for all the Cantonese characters. You often see this like o的,o個,o即 instead of 啲,嗰,嘅. I know a lot of people in Hong Kong use Cangjie for character input and a lot of the Cantonese characters don't have (or didn't have) Cangjie mappings. >> I wonder how this affects written communication for those who don't speak Mandarin natively When speaking Taiwanese it is common to use Mandarin words for more educated topics or if you don't know the word in Taiwanese just use the Mandarin term. Probably because Taiwanese is not generally used in higher education and there are varying levels of Taiwanese fluency in Taiwan. I would imagine it's similar for a lot of dialects in China since speaking dialects is often discouraged and a lot of younger people abandon their native dialects.