Basically according to Ethnologue, there about 6500 languages in the world. I don’t know how many exactly, but probably no more than 1000 have a writing system. By the way, how many languages can you think of? Probably not more than one or two hundred at most, and those of course are all ones with writing systems. Usually it is those Bible tarnslators traveling into the jungles and giving these languages writing systems for the first time, and of course it’s always in a romanized script.
And by the way, Cantonese and Minnan are considered by the linguists of Ethnologue to be separate languages based on statistical and historical data (including other languages of China as well).
Taiwanese (= Minnan = Hokkien) is the same dialect as spoken in Xiamen. There are two main accents: Zhangzhou and Quanzhou. Here in Taipei, most people speak with Quanzhou (for example the word [勿會] “cannot” (the two characters should be merged together) is pronounced ‘be’ in Quanzhou accent and ‘boe’ in Zhangzhou accent, and more or less mutually intelligible).
Several Romanizations already exist for writing Taiwanese. In addition, there are at least two differing adaptations of Zhuyin Fuhao for Taiwanese, and there are Zhuyin Fuhao for Hakka as well. It wouldn’t take an expert to figure out that ㄍwith a curl on it is Taiwanese’ breathy-voiced ‘g’, especially if it’s a language you already speak.
Many, many children learn Taiwanese as a first language at home before they ever get to school. Of course there are many, many parents that speak to them only in Mandarin so they won’t feel burdened once they get to school, but if they’re coming from a Taiwanese family, chances are those children have had more exposure to Taiwanese than Mandarin by the time they start school.
I don’t think it is the goal of schools to teach children Taiwanese since many already speak it (those that come from Taiwanese-speaking families). It is rather expanding their vocabulary, giving them a tool to record the speech in writing, and better their ability to discuss things in the language. 遮兮(个)代誌毋是赫呢歹了解 (chit-e taichi m-si hiah-ni phaiN liaokai).
啄齒 means teeth and 啄 means mouth. 出去 means to go out. As mentioned before they have different pronunciations and different tones. As a side note 恬去 (tiam7 khi3) means to shut up, and it is not 齒 khi2 here but 去 khi3.
Personally, I prefer typing Taiwanese in romanization because I can do it so much faster than characters, but if I need to type in characters, I can always find a character for what I need.
There are some useful characters that can’t be typed, and I usually just put their components in such as [勿會], [勿愛], [亻因], [辶日]迌 and so on. Here is a site with some characters used in Taiwanese:
home.kimo.com.tw/kiatgak/choji.htm
Kasu kong, chia u lang siuN ka goa liansip Taigi, goa ma oan-gi. In-ui goa ka-ki e Taigi a-boe hiah-ni ho, so-i goa hi-bang chia u lang siuN liansip.