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Miranda IM is a open source multi-protocol instant messaging client. Miranda IM is Very light on system resources and extremely fast. Miranda has many plugins, and You can customized Miranda IM client what you want.
This Project make a Japanese Langage package for Miranda IM.
Links * Miranda IM official webpage
You can download japanese langage package from the following site. Miranda IM Japanese langage package
You can download file releases of Miranda IM Japanization project from List of release files
System RequirementsOperating System: Windows
List of release files