Language support for stanza content

[Jajcus]

Currently language is only negotiated and set for the stream root. In XMPP any XML element containing readable text can have a language specified and usually can be provided in many language versions. Details are described in RFC 6120.

In particular the following elements can have alternate languages:

• Stream error and stanza error elements
• Message stanza and
• Presence stanza

All these should be handled with the same API, probably using some 'multi-language string' type instead of plain unicode for appropriate properties.