Enable a bill-template language for the firm.
firm_language row for the given ISO-639-1 two-letter code. The
server lowercases the input and validates:
400 invalid_code — code is not two alphabetic characters.400 unsupported_language — code is not in static_language.created: false.Use your API key (sk_live_xxx or sk_test_xxx)
Two-letter lowercase ISO-639-1 code.
"en"