It is not possible to create Custom Language Codes in Cloud for Customer
SAP Cloud for Customer
Reproducing the Issue
Language code does not have code list mapping support due to technical limitation ( SAP stores the language key internally as a single character for the reason of memory optimization and ISO codes(2 digit) is used for communication)
C4C strictly follows ISO codes for language codes and the option for code list mappings is not available, hence, whatever values are sent in the payload are considered directly. And since the system strictly follows ISO codes, this is why there is not an option to create new language codes in C4C.
So you have to use the languages supported (Those available under Additional Communication Languages in Business Configuration).
KBA , LOD-CRM-IND-UTL , Utilities , Problem