The language of your book should be defined in two places:
The metadata section of the OPF:
<metadata xmlns="http://www.idpf.org/2007/opf" xmlns:dc="http://purl.org/dc/elements/1.1/" >...<dc:language>en</dc:language>...</metadata>Language abbreviations can be found in the language subtag registry (http://www.iana.org/assignments/language-subtag-registry).
In the metadata provided to Apple (the language metadata provided to Apple must be the same language as specified in the OPF).
Note: For Chinese language books, you must specify both the language (zh) and the script (Hans or Hant) portions of the language code. The language code for Simplified Chinese is zh-Hans and the code for Traditional Chinese is zh-Hant. The code must be specified in both the book’s OPF file and in the metadata using the <language> tag. See Apple Books Metadata XML File Annotated in the Apple Books Book Package Specification for information on the <language> tag.