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The problem is that at it's core, the ResourceBundle API uses Properties#load(InputStream) method to load the properties files. This method uses by default the ISO-8859-1 encoding. This is explicitly mentioned in its javadoc as well,
The load(InputStream) / store(OutputStream, String) methods work the same way as the load(Reader)/store(Writer, String) pair, except the input/output stream is encoded in ISO 8859-1 character encoding. Characters that cannot be directly represented in this encoding can be written using Unicode escapes ; only a single 'u' character is allowed in an escape sequence. The native2ascii tool can be used to convert property files to and from other character encodings.
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References
Good overview of problem and possibly good solution for jsf - http://jdevelopment.nl/internationalization-jsf-utf8-encoded-properties-files/