Glad to know you're still working on this Jabier,

Some comments:

Realy Im not sure if "requiredFeatures" and "requiredExtensions" are
useful to handle on Inkscape or maybe is better for use on the XML
editor or raw SVG editing for the moment.

I would say that the requiredFeatures/extensions attributes are not useful.  They were not very well defined in the original SVG specs, and are inconsistently implemented in browsers.
 
In my mind, with language only point of view, I need a object dialog
method to redraw a "switch", and a language -or "switch"- widget to:
        * Add multilenguage title and desc
        * Switch all "switches" to one lang
        * Add lang to a "switch". Global?
        * Remove a lang
        * Insert selected element to a new "switch" in the default lang.
        * Handle more Aria atributes?

When *displaying* content, you only use one language at a time for the whole graphic.   So for the user interface, you need a single option to switch between languages.  I think it would make the most sense to put it within the "View" menu, to make it clear that you are not changing the document, you are changing the way you look at the document.

For defining the language alternatives, I think for a start it would be easiest to focus on switching individual text elements or individual title/desc elements.  Allowing switches to make large scale changes to the graphics and layout would probably cause to much complication.

So the place to start would be the object properties dialog (for title/desc) and the text properties dialog. Things you would need:
  • A drop-down list to declare the language of the content.  
  • An "Add language" button which inserts another text field & language drop-down
  • If there are more than one languages, some sort of radio-button option for indicating which one is the default.
The following things would be nice extras, but aren't essential:
  • Make the language selection a two-part drop down for main language (English, Spanish) versus country code variation (Canadian English vs US English vs UK English vs any English)
  • A way to re-order the languages.  The browser will use the first language that matches one of the user's preferences, so the order matters.
Not that the way for expressing the default in the XML is different: for title/desc, the first option is default.  It will also be used if the browser does not support the new SVG 2 language switching feature (Right now, I don't think any browser supports it!).  For <switch>, the last option is the default, and it cannot have a "systemLanguage" attribute (although it should still have a xml:lang attribute).

Quick demo of the different syntax.  The English version is the default, but if your browser is set to use Spanish or French, you should see the visible text in that language.  Let me know if you find a browser that also switches the tooltip!

http://fiddle.jshell.net/rzz2ac4g/1/



By the way, about "switches" in webpages with "systemLanguage"
conditional processing attribute, whats the best way to user based
switch a lang? We need a JS hack?

Yes, currently there would need to be a JS hack to allow the user to see a different language without having to change their browser language settings.  You would need to change the DOM to re-order the elements and remove the systemLanguage attribute.

 
Now im very bussy whith my payed work but hope can take some time to
this proyect but sure not as dedicated as previously :(
 
I'm pretty sure everyone understands this!