To encode the locale on any given XML packet, we use the xml:lang attribute, as defined in the XML specification. This in turn uses values as specified in RFC 1766 to encode languages and regions. This way, you can even distinguish between British and Australian English.

An xml:lang tag can be put onto any XML element; for the purposes of this document, however, we will limit its usage to the four central Jabber elements: <stream/>, <message/>, <iq/> and <presence/>.

A client claiming to support this document has to initiate server connection slightly differently by putting an xml:lang attribute in the initial <stream:stream> element.

Servers not supporting this document will just ignore the additional attribute. Compliant server can be distinguished by the fact that their reply <stream:stream> element also contains an xml:lang attribute, indicating the main language of the server. A compliant client has to detect whether the server is compliant or not, and base its future behavior on this information.

If the client thus determines that the server is compliant, then it doesn't have to do anything beyond this point. All its outgoing messages will automatically be flagged by the server with an xml:lang attribute if necessary. Thus writing a minimal compliant client is trivial.

If it is determined that the server does not support this document, and the client still wants to offer locale support, it may start flagging all its outgoing message/iq/presence elements with the xml:lang attribute, to ensure that other components/clients which do conform to this document can handle the localization despite the local server not doing so.

Finally, if for whatever reasons the client wants to flag particular messages with a different locale (e.g. if the user is bilingual), it can do so at any time by putting an appropriate xml:lang element in the outgoing data. This will override the previously set default locale for this message only.

A compliant server must detect the xml:lang attribute in incoming <stream:stream> elements. The server then has to store this information for later use, i.e. it has to remember the default language for each active session.

Additionally, a compliant server must attach an xml:lang attribute to the reply <stream:stream> element sent in response to a newly initiated connection. This attribute should reflect the default language of that server, and is used to indicate to clients that the server implements this document.

The server should not only allow user clients to specify a default language this way, but also server-side components, like the JUD should be allowed to do this.

Whenever a message leave the server, it has to tag the message automatically with the xml:lang attribute of the corresponding seesion, if any was specified, unless the message is already tagged this way. In that case, the already existing xml:lang attribute takes precedence, thus allowing for greater flexibility.

If a client send a message to another local client which uses the same xml:lang value, then no change is applied. But if the recipient uses a different xml:lang, and if the message has no xml:lang attribute attached yet, the xml:lang of the server has to be attached before delievey of the message.

Jabber based services that wish to comply to this document have to make sure that all information they send to clients is tagged with an xml:lang attribute corresponding to the language used in the outgoing data, if appropriate, even if the component supports no other localizations. An example for this is a search form based on XEP-0004.

This way, a client could for example offer to translate the form since it now knows the language the form was written in. Previously it could just guess the language was English, which never was guaranteed.

To be able to tailor replies to the user's preferred language, the component has to know this information. This is simply inferred from any xml:lang attribute on incoming requests. If none is present, the default locale is assumed. If the client's default locale diverges from that of the component, it is the server's responsibility to tag the query with an appropriate xml:lang attribute (refer to the "Server support" section). If on the other hand the server is not compliant, then any interested client will manually tag its queries with an xml:lang attribute. Thus it is sufficient to check for this attribute.

A more sophisticated component supporting multiple localizations of its forms/messages could now honor the requested language and send this search form instead of the English one shown previously:

If the component doesn't have the requested localization available, it replies with the default localization (but of course with the matching xml:lang attribute tagged to it, and not the one of the request).