I don’'t speak Japanese but I did try the openfire_i18n_ja_JP.properties(.converted) file and did add it with
jar cf ja.jar openfire_i18n_ja_JP.properties
to a new jar file. Placing this file in the lib directory and changing the locale to ja_JP did change the language completely as far as I can tell (*1). For me this seems to be more easy than the steps you did describe in your post.
Maybe you can confirm this, this would make it more easy for other users to add it.
LG
(*1) As far as I know Openfire reads all jar files in the lib directory in alphabetical order. So if you want to replace some classes of property files within Openfire.jar you may simply add “N.jar” to the lib directory and it will be read before. As you just want to add a new translation the jar file may have a random name like ja.jar as it does not really matter when it is read.