Messenger’'s support for i18n in mainly restricted to server messages. Messenger ships with a default english message set that you can specialize for your language by adding a file to the classpath named jive_i18n_xx.properties with xx being your language. You can see the jive_i18n_en.properties file in the lib/jive-xmpp.jar for the messages that should be translated.
When communicating between clients, Messenger simply relays the text it receives (it doesn’'t attempt to do any encoding/decoding of text passing through it). Clients must encode their text in UTF-8 then properly decode it on the other end.
This is a client issue that unfortunately can’‘t be resolved by Messenger. Think of XMPP IM like email. In email, the server doesn’‘t do anything to the messages, it simply receives them from the sender, delivers them to the correct user, and allows that user to retrieve them. It’‘s up to the clients to encode and decode the messages properly. XMPP works the same way; the server only delivers messages - it’'s up to the clients to encode and decode them properly.