The best suggestion for foreigners going to Taiwan is learn Mandarin first then learn Taiwanese in Taiwan. Taiwanese , also known as Southern Min Chinese or Min Nan is different from Mandarin Chinese in terms of spoken form, expressions and syntax. Generally speaking , Taiwanese is more difficult to learn than Mandarin not only because of lesser resources, but also the nature of the language itself. Taiwanese has seven to eight tones while Mandarin has four tones. Taiwanese has colloqual and literary pronounication for chinese characters, and some chinese characters have more than two readings which is confusing for beginners, and there are tone sandhi rules which is rather complex. The best way to learn Taiwanese for foreigners would be  going to Taiwan , attend Taiwanese courses conducted by senior Taiwanese teachers and talk to middle aged and elderly Taiwanese people who speak Taiwanese. Currently, Mandarin is more widely spoken in Taiwan than Taiwanese. However, it would be a note of interest that Taiwanese Language would be somewhat slightly more widely spoken in Southern Taiwan. Unfortunately, there are many young and some middle aged Taiwanese people who prefer to speak Mandarin than their so-called Taiwanese mother tongue. In my humble opinion as well as Taiwanese Language advocates, some of the young, some of the middle aged and elderly generations of Taiwanese who speak Taiwanese , they are worried that the widespread use of Mandarin would damage the existence of Taiwanese Language.

Mandarin Chinese is enough.  My Taiwanese sucks and I've gotten by fine here for most of my life.

I used to intern at a hospital in Taipei.  For two weeks, I manned the reception desk at the diagnostic department and I talked to a lot of elderly people all day.  There were a few who didn't speak Mandarin and I had to utilize my poor Taiwanese to communicate.  The more south you go, the more Taiwanese is spoken.  You may find more elderly people in the south who only speak Taiwanese.  Young people always know Mandarin, no matter how south you go.  So unless you're studying something that requires you to speak to a lot of elderly people, you should be fine knowing only Mandarin. 

After the Martial Law ended in Taiwan and the start of democratisation of Taiwan, although the Democratic Progressive Party stepped in to so-called promote and liberalise the use of Taiwanese Language, Taiwanese Culture and Language are not fostered enough beyond lip service of these Pro-Independence Taiwanese Politicans who made use of Taiwanese Language as a cynical ploy to win seats in Taiwanese elections. The Taiwanese Language advocates, some young , some middle aged and elderly who speak Taiwanese Langauge who are likely to be Pro Independence as well, are saying that Taiwanese is an endangered Langauge in Taiwan, they say that if the serious problem of Taiwanese persists, Taiwanese Language may be well extinct in about 20 years. By then, it is time to bid farewell to Taiwanese Language and the Taiwanese Language Books and Taiwanese Language Podcasts be kept as historical artifacts to be stored in the museum for display!!!

I would not say that it is worthless to learn Taiwanese Language in Taiwan. Of course there are testimonals of Foreigners residing in Taiwan who managed to pick up Taiwanese during their residence in Taiwan. It was rather surprising that some of the foreigners spoke Taiwanese more fluently than the local Taiwanese. Some of the foreigners who picked up Taiwanese in Taiwan on their own accord believed that they should learn Taiwanese Langauge to understand Taiwan at a deeper level, and that they believe that it was normal to be speaking Taiwanese in Taiwan and they hope to inspire and influence the local Taiwanese into speaking Taiwanese.

You got to understand that Taiwanese had a history of being heavily suppressed by Kuomingtang Political Party since they took over Taiwan in 1945, causing in the decline of Taiwanese for many decades. To make matters worse Kuomingtang have been spreading propaganda about Taiwanese Language, calling Taiwanese as low class language in favour of promoting Mandarin , causing many Taiwanese to discriminate Taiwanese Langauge till today. I heard of some stories of some overseas chinese and foreigners who speak Taiwanese in Taiwan being questioned by the Local Taiwanese people as being Overseas Taiwanese, Chinese Malaysians, Chinese Singaporeans, Chinese Indonesians who speak Taiwanese. Sadly, many local Taiwanese have been heavily brainwashed by Kuomingtang into discriminating Taiwanese Language, it has become an awkard situation whereby some foreigners in Taiwan and some overseas chinese who embrace and have no qualms about speaking Taiwanese, while the local Taiwanese don't want to speak Taiwanese themselves. In general, many local Taiwanese are not proud of speaking Taiwanese Language and they are not confident of the Taiwanese Language and Culture. Many local Taiwanese felt that they ought to be ashamed of speaking Taiwanese Language, Taiwanese Language is uncivilised they say, that was what the Kuomingtang Political Party have been instilling into the local Taiwanese people minds for decades which still persist today.

Hi Wei, Sharon, and Seth

Thank you so much for your advices! I do have problems finding materials to learn Taiwanese, it's a relief to know that most people speak Mandarin. I'll learn Mandarin then :)