Hi Taehyung,
At this time, because AtoM is using an older PHP framework (Symfony 1x), there a couple things to note here:
First, AtoM is actually currently using the ISO-639-1 two-letter langauge codes, not the 639-2 three-letter ones.
Second, we maintain a list of all supported cultures in the following gist:
This includes the English name of the supported culture, as well as the related code. Note that only *some* sublocales are supported in Symfony - for example, we have support for Brazilian Portuguese (pt_BR), but not ur_PK - Urdu (Pakistan) - unfortunately, Symfony only has support for a general Urdu (ur) language at the moment.
Finally, because this is embedded in the Symfony framework, there is no way to upgrade the language support in AtoM without development work to patch the framework.