Gajim should set the xml:lang attribute on the stream root to get native-language responses from the server
RFC 3920, section 4.4 states, that an xml:lang attribute SHOULD be included on the stream root element. Gajim currently does not set this attribute. I would suggest sending this attribute, because it allows the server to customize its responses to the language, that is requested by the client. E.g. error messages can then be returned in the language of the client.