Returns three-letter ISO-639-2 "terminological" language code given a
two-letter ISO-639-1 language code or a three-letter ISO-639-2 language
code (both are accepted for convenience).
The "terminological" code is derived from the local name of the language
(e.g. "deu" for German instead of "ger"). In most scenarios, the
"terminological" codes are preferred over the "bibliographic" ones.
Language codes are case-sensitive and expected to be lower case.
Returns three-letter ISO-639-2 "terminological" language code given a two-letter ISO-639-1 language code or a three-letter ISO-639-2 language code (both are accepted for convenience).
The "terminological" code is derived from the local name of the language (e.g. "deu" for German instead of "ger"). In most scenarios, the "terminological" codes are preferred over the "bibliographic" ones.
Language codes are case-sensitive and expected to be lower case.