UTS #35: Unicode Locale Data Markup Language (LDML) is the specification of the XML format used for CLDR data, including the interpretation of the CLDR data.
Definitions - common definitions used in CLDR
Unicode Locale Extension (‘u’) for BCP 47 - an overview of the -u- extension
CLDR Survey Tool - how to use the Survey Tool
Coverage Levels - Detailed descriptions of the data required for a language to be considered supported at a specific level
Unicode Transliteration Guidelines - how to design and use Unicode transliterators
Picking the Right Language Identifier - how to pick the right language code
Collation Guidelines - how to construct collation rules