Taiwan
In Taiwan, Mandarin Chinese is the official language. Even if many people will speak the Taiwanese dialect, they normally also speak Mandarin. Taiwan still used the traditional characters. Reading will thus be difficult if your learnt the simplified.

Singapore
Mandarin is an official language of Singapore and the simplified characters are used. This is the most widely spoken Chinese language spoken, even if some Cantonese dialects are also used. English is however much more widespread than in China or Taiwan, so the usefulness of Mandarin Chinese is reduced.

Hong-Kong
Even if the use of Mandarin Chinese is increasing in Hong-Kong, the basic Chinese language spoken there is Cantonese, not Mandarin. English is also widely spoken and people will not think natural to speak Mandarin with Westerners.