Dear Isobel,
I knew that you could put SVG in docbook, but I wasn't really thinking about how that might impact language switching. I also had not thought about using XSL as a language switch, although it is a natural use of that technology.
I don't think there should be any conflict with using SVG <switch> inside docbook, since this is a standard SVG element -- the question is whether your docbook processor has implemented support for it! If you want to test out any tools with Jabier's sample graphic, I'd love to hear the results.
The switching for <title> and <desc> elements might interact with docbook & XSL switching, since they all use the basic lang attribute. However, I don't think it would be a conflict, it would just mean that the language switching might happen at a different stage (when generating the styled Book XML instead of when rendering the SVG to an accessible form).
If a DocBook processor doesn't support SVG <switch>, the other option would be to generate separate, complete versions of the SVG, and include them as alternate media objects with docbook language switchers. Which is another use case for having an "Export in Language" option for Inkscape.
Thanks for the input,
Amelia