Returns two-letter ISO-639-1 language code given a three-letter ISO-639-2 language code or two-letter ISO-639-1 language code (both are accepted for convenience).
Language codes are case-sensitive and expected to be lower case.
ISO-639 language code (e.g. "deu" or "ger" or "de")
two-letter ISO-639-1 language code string that maps to lang_code, or NULL if no mapping is known. The returned string must not be modified or freed.
Returns two-letter ISO-639-1 language code given a three-letter ISO-639-2 language code or two-letter ISO-639-1 language code (both are accepted for convenience).
Language codes are case-sensitive and expected to be lower case.