Hello,
I have recently succeeded externalising the messages.properties file.
I set cas.messageBundle.baseNames in my application.properties file as:
cas.messageBundle.baseNames = file:/path_to_newfile,classpath:custom_messages,classpath:messages
the path_to_new_file should not have the file extension .properties, as spring boot will look for _<lang>.properties files on that directory. So for example if your new file is custom_messages_en.messages, then you only need to set file:/path_to_custom_messages.
Hope that helps.
Kind regards