Language the campaign operates in: en_ca (Canadian English), en_gb (British English), en_us (American English), de (German), es (Spanish), fr (French), it (Italian), nl (Dutch), nl_be (Flemish), pt (Portuguese), pt_br (Brazilian Portuguese), zh_hk (Chinese), ar (Arabic), ja (Japanese), ko (Korean), ru (Russian), pl (Polish), tr (Turkish), hu (Hungarian), sv (Swedish), no (Norwegian), da (Danish). When omitted, it is derived from the language of openingMessageTemplateId, then from the organization's language when it supports only one, and finally defaults to en_us. WHATSAPP campaigns are rejected with a 400 when the language has no approved WhatsApp templates for the organization, because WhatsApp follow-up messages require a pre-approved template in the campaign language; set disableNudging to true to opt out of follow-ups and skip that check.