Hi Seba
Yann Perrin (fr) made a switch-language plugin some years ago :
http://dl.dropbox.com/u/3105342/TW/twkd.html#PolyGlotPluginDoc_en
However this plugin isn't compatible with TiddlyWiki ver 2.6
I see no reason why not to take a shot at building something similar
with different tools available from TiddlyTools.com (Eric Shulman's
pluginheaven)
As a startingpoint I would install TemplateTweak
http://www.tiddlytools.com/#TaggedTemplateTweakInfo and create custom
view and edittemplates for each language (set by tags)- all language
entries could be sections input into the same tiddler via different
edittemplates if you integrate Erics EditSectionPlugin
http://www.tiddlytools.com/#EditSectionPlugin in the ViewTemplates,
you might just skip making custom edittemplates because it lets you
edit individual sections, when in viewmode.
Another strategy is to install HideWhenPlugin
http://mptw.tiddlyspot.com/#HideWhenPlugin
and set it up to hide different classes(languages for your purpose)
depending on tagvalues - in the "standard" ViewTemplate.
For switching between different languages (invoked by tagvalue) I
would definetely install Tobias Beer's CycletagPlugin to be able to
roll from one language to another, not having to write anything...
http://lastfm.tiddlyspot.com/#cycleTags NB: You'll need Eric's
inlineJavascriptPlugin to make it work
There are many, many more ways to do it - however these are some of
the Plugins I use every day -
I might give it a shot myself one of these days - summerholliday
etc...
Happy hacking
Regards Måns Mårtensson