I'm just completing a new feature that lets advertisers on a site specify which languages they can speak. For Chinese I'm not sure what to put.
Which language(s) can you speak? - English, - Spanish - French - Japanese - Chinese
For Chinese, is that sufficient? I (think I) know there are Mandarin and Cantonese. So can these two understand each other? Do most people who travel speak one by default, or both? What is the best practice here?
'Chinese' will be assumed to be Mandarin, which is very much the official language - it's also an official language in Hong Kong, but even then a lot happens in Mandarin. I'd just leave it as 'Chinese' - apart from anything else if their English isn't good they might not know the word Mandarin.
>> Do most people who travel speak one by default, or both? What is the best practice here?
Most people in Mainland China / Taiwan will speak something closer to Mandarin (or Putonghua). In the southern regions, closer to the Canton / Guangdong provinces you'll find Cantonese or (Guangdonghua ) more popular as is the case here in Hong Kong.
Verbal Chinese can get far more confusing because of very strong regional accents. Shanghainese can have a hard time understanding someone from Beijing even though they're speaking the same language.
You also have to deal with regional dialects like Hokkien etc.
Life is much more simpler if you have to ask what script they can read - Simplified or Traditional.