You do not need to configure any custom messages for WMS to be able to send the messages in a different language. WMS SIP2 determines which language to use in its response when the initial patron request is made. The language controls translations for screen messages, date/time formatting, etc.
You will need to configure your SIP2 client to send one of the following SIP2 language codes supported by WMS:
If your client sends a different code [e.g. 011 for French (Canadian)], WMS defaults to English (U.S.). If you believe you've configured your client correctly, but WMS is still sending the incorrect language, please contact OCLC Support with your OCLC Symbol and the SIP2 logs from an example transaction with the incorrect language.