language

The BCP 47 tag of the language the text is written in, such as ja, or null when the document does not say. One Han character has a Japanese, a Simplified Chinese, a Traditional Chinese and a Korean form, and a generic serif face draws only one of them, so a substitute for a CJK font needs a face of its own language. A PDF CIDFont names it through the character collection of its CIDSystemInfo (ISO 32000-1, 9.7.3, #472): Adobe-Japan1 is ja, Adobe-GB1 zh-Hans, Adobe-CNS1 zh-Hant and Adobe-Korea1 ko.