Since the question is tagged C# and .NET 3.5 I'm assuming the following .NET-specifics apply.
There's a hint on this page zh-Hans, zh-Hant and the "old" zh-CHS, zh-CHT
use the IETF standard "zh-Hans", and "zh-Hant" names for Chinese simplified and traditional ... in .Net 2.0/3.x we still recognize zh-CHS & zh-CHT for backwards compatibility ... LCID (0x0004 or 0x7C04)
And from W3 I18N FAQ
"zh-Hant" and "zh-Hans". These language codes do not represent specific languages. "zh-Hant" would indicate Chinese written in Traditional Chinese script. Similarly "zh-Hans" represents Chinese written in Simplified Chinese script. This could refer to Mandarin or many other Chinese languages.
I guess the key point is that Chinese Mandarin that you mentioned is a spoken dialect (for want of a better term), and Chinese (Simplified) is the 'character set' that is used to represent that spoken language 'on paper'. LocaleId 0x0004 according to coobird's link.
Other spoken languages (Cantonese?) are represented by different 'character sets' like Chinese (Traditional). If your application was going to Taiwan or Hong Kong you might use LocaleId 0x7C04.
- NOTE: I'm not meaning 'character set' in the computerese/ASCII sense above ~ just distinguishing between the spoken and written representations. There's a whole other discussion on character sets/codepages/unicode (although generally .NET seems to cope with that stuff as long as your fonts are sorted out and you stick to UTF-8).