CSV can have headers (email, first_name, last_name, tags, language) or just data without headers.
Language defaults to 'en' if not specified. Supported languages: en, zh (Chinese)
With headers (including language):
email,first_name,last_name,tags,language
john@example.com,John,Doe,"newsletter,tech",en
wang@example.com,Wei,Wang,"newsletter",zh
With headers (language will default to 'en'):
email,first_name,last_name,tags
john@example.com,John,Doe,"newsletter,tech"
Without headers (3 columns):
John,Doe,john@example.com
Jane,Smith,jane@example.com