The BASICTrans module provides support to BASIC and BASIC64 modules so that messages can be translated for use in another territory. The BASIC interpreter issues calls to the BASICTrans module which is responsible for providing messages to the particular territory.
By replacing one BASICTrans module with another, you can change the language used by BASIC for its messages.
Both BASIC and BASIC64 issue the same calls to the same BASICTrans module, thus code and messages are shared between the two modules.