Via a post by Amir Elisha Aharoni on the paradox of localization:

People
who don’t know English strongly prefer to use software in a language
they know... People who do know
English often prefer to use software in English even if it is available
in their native language. The two most frequent explanations for that
is that the translation is bad and that people who want to use computers
should learn English anyway. The problem is that for various reasons
lots of people will never learn English even if it would be mandatory in
schools and useful for business. They will have to suffer the bad
translations and will have no way to fix it.

So this is the paradox – to fix localization bugs, someone must
notice them, and to notice them, more people who know English must use
localized software, but people who know English rarely use localized
software... Even people
who know English well should use software in their language... especially
if it’s translated badly, because they are the only ones who can report
bugs in the translation or fix the bugs themselves...

I am glad to say that i convinced most people to whom i spoke about
it at Wikimania to at least try to use Firefox in their native language
and taught them where to report bugs about it. I also challenged them to
write at least one article in the Wikipedia in their own language, such
as Hindi, Telugu or Kannada – as useful as the English Wikipedia is to
the world, Telugu Wikipedia is much more useful who speak Telugu, but no
English.

My comment,
summarized: can the software be hacked to make this easy for
multilingual users to contribute to? Imagine being asked to opt-in for occasionally
having the application start up in (or switch into) another language
you know -- for instance, "every 25th time you log into this website,
you'll see it in Hindi" -- with a reminder shown during those
language-switching times thanking you for aiding with translation
quality and giving you a direct link to report translation problems?

Because sometimes the problem is just a little too much friction;
switching the language of an application takes 5 too many mouse-clicks
to be thoughtlessly easy, and the vast majority of users don't wake up
in the morning and think "you know, I could contribute to the localization of the software on my computer!" The lazier people can be and still help you, the more likely they'll be to do it.

Which applications, or which types of applications, would be easiest to do this with?